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llvm-project/llvm/test/CodeGen/X86/cmov-into-branch.ll

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[x86] auto-generate checks for cmov tests llvm-svn: 267417
2016-04-25 23:26:57 +08:00
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: llc -mtriple=x86_64-unknown-unknown < %s | FileCheck %s
CodeGenPrepare: Add a transform to turn selects into branches in some cases. This came up when a change in block placement formed a cmov and slowed down a hot loop by 50%: ucomisd (%rdi), %xmm0 cmovbel %edx, %esi cmov is a really bad choice in this context because it doesn't get branch prediction. If we emit it as a branch, an out-of-order CPU can do a better job (if the branch is predicted right) and avoid waiting for the slow load+compare instruction to finish. Of course it won't help if the branch is unpredictable, but those are really rare in practice. This patch uses a dumb conservative heuristic, it turns all cmovs that have one use and a direct memory operand into branches. cmovs usually save some code size, so we disable the transform in -Os mode. In-Order architectures are unlikely to benefit as well, those are included in the "predictableSelectIsExpensive" flag. It would be better to reuse branch probability info here, but BPI doesn't support select instructions currently. It would make sense to use the same heuristics as the if-converter pass, which does the opposite direction of this transform. Test suite shows a small improvement here and there on corei7-level machines, but the actual results depend a lot on the used microarchitecture. The transformation is currently disabled by default and available by passing the -enable-cgp-select2branch flag to the code generator. Thanks to Chandler for the initial test case to him and Evan Cheng for providing me with comments and test-suite numbers that were more stable than mine :) llvm-svn: 156234
2012-05-05 20:49:22 +08:00
[CodeGenPrepare] Remove load-based heuristic Summary: Both the hardware and LLVM have changed since 2012. Now, load-based heuristic don't show big differences any more on OoO cores. There is no notable regressons and improvements on spec2000/2006. (Cortex-A57, Core i5). Reviewers: spatel, zansari Differential Revision: http://reviews.llvm.org/D16836 llvm-svn: 261809
2016-02-25 08:23:27 +08:00
; cmp with single-use load, should not form branch.
CodeGenPrepare: Add a transform to turn selects into branches in some cases. This came up when a change in block placement formed a cmov and slowed down a hot loop by 50%: ucomisd (%rdi), %xmm0 cmovbel %edx, %esi cmov is a really bad choice in this context because it doesn't get branch prediction. If we emit it as a branch, an out-of-order CPU can do a better job (if the branch is predicted right) and avoid waiting for the slow load+compare instruction to finish. Of course it won't help if the branch is unpredictable, but those are really rare in practice. This patch uses a dumb conservative heuristic, it turns all cmovs that have one use and a direct memory operand into branches. cmovs usually save some code size, so we disable the transform in -Os mode. In-Order architectures are unlikely to benefit as well, those are included in the "predictableSelectIsExpensive" flag. It would be better to reuse branch probability info here, but BPI doesn't support select instructions currently. It would make sense to use the same heuristics as the if-converter pass, which does the opposite direction of this transform. Test suite shows a small improvement here and there on corei7-level machines, but the actual results depend a lot on the used microarchitecture. The transformation is currently disabled by default and available by passing the -enable-cgp-select2branch flag to the code generator. Thanks to Chandler for the initial test case to him and Evan Cheng for providing me with comments and test-suite numbers that were more stable than mine :) llvm-svn: 156234
2012-05-05 20:49:22 +08:00
define i32 @test1(double %a, double* nocapture %b, i32 %x, i32 %y) {
[x86] auto-generate checks for cmov tests llvm-svn: 267417
2016-04-25 23:26:57 +08:00
; CHECK-LABEL: test1:
; CHECK: # BB#0:
; CHECK-NEXT: ucomisd (%rdi), %xmm0
; CHECK-NEXT: cmovbel %edx, %esi
; CHECK-NEXT: movl %esi, %eax
; CHECK-NEXT: retq
;
[opaque pointer type] Add textual IR support for explicit type parameter to load instruction Essentially the same as the GEP change in r230786. A similar migration script can be used to update test cases, though a few more test case improvements/changes were required this time around: (r229269-r229278) import fileinput import sys import re pat = re.compile(r"((?:=|:|^)\s*load (?:atomic )?(?:volatile )?(.*?))(| addrspace\(\d+\) *)\*($| *(?:%|@|null|undef|blockaddress|getelementptr|addrspacecast|bitcast|inttoptr|\[\[[a-zA-Z]|\{\{).*$)") for line in sys.stdin: sys.stdout.write(re.sub(pat, r"\1, \2\3*\4", line)) Reviewers: rafael, dexonsmith, grosser Differential Revision: http://reviews.llvm.org/D7649 llvm-svn: 230794
2015-02-28 05:17:42 +08:00
%load = load double, double* %b, align 8
CodeGenPrepare: Add a transform to turn selects into branches in some cases. This came up when a change in block placement formed a cmov and slowed down a hot loop by 50%: ucomisd (%rdi), %xmm0 cmovbel %edx, %esi cmov is a really bad choice in this context because it doesn't get branch prediction. If we emit it as a branch, an out-of-order CPU can do a better job (if the branch is predicted right) and avoid waiting for the slow load+compare instruction to finish. Of course it won't help if the branch is unpredictable, but those are really rare in practice. This patch uses a dumb conservative heuristic, it turns all cmovs that have one use and a direct memory operand into branches. cmovs usually save some code size, so we disable the transform in -Os mode. In-Order architectures are unlikely to benefit as well, those are included in the "predictableSelectIsExpensive" flag. It would be better to reuse branch probability info here, but BPI doesn't support select instructions currently. It would make sense to use the same heuristics as the if-converter pass, which does the opposite direction of this transform. Test suite shows a small improvement here and there on corei7-level machines, but the actual results depend a lot on the used microarchitecture. The transformation is currently disabled by default and available by passing the -enable-cgp-select2branch flag to the code generator. Thanks to Chandler for the initial test case to him and Evan Cheng for providing me with comments and test-suite numbers that were more stable than mine :) llvm-svn: 156234
2012-05-05 20:49:22 +08:00
%cmp = fcmp olt double %load, %a
%cond = select i1 %cmp, i32 %x, i32 %y
ret i32 %cond
}
; Sanity check: no load.
define i32 @test2(double %a, double %b, i32 %x, i32 %y) {
[x86] auto-generate checks for cmov tests llvm-svn: 267417
2016-04-25 23:26:57 +08:00
; CHECK-LABEL: test2:
; CHECK: # BB#0:
; CHECK-NEXT: ucomisd %xmm1, %xmm0
; CHECK-NEXT: cmovbel %esi, %edi
; CHECK-NEXT: movl %edi, %eax
; CHECK-NEXT: retq
;
CodeGenPrepare: Add a transform to turn selects into branches in some cases. This came up when a change in block placement formed a cmov and slowed down a hot loop by 50%: ucomisd (%rdi), %xmm0 cmovbel %edx, %esi cmov is a really bad choice in this context because it doesn't get branch prediction. If we emit it as a branch, an out-of-order CPU can do a better job (if the branch is predicted right) and avoid waiting for the slow load+compare instruction to finish. Of course it won't help if the branch is unpredictable, but those are really rare in practice. This patch uses a dumb conservative heuristic, it turns all cmovs that have one use and a direct memory operand into branches. cmovs usually save some code size, so we disable the transform in -Os mode. In-Order architectures are unlikely to benefit as well, those are included in the "predictableSelectIsExpensive" flag. It would be better to reuse branch probability info here, but BPI doesn't support select instructions currently. It would make sense to use the same heuristics as the if-converter pass, which does the opposite direction of this transform. Test suite shows a small improvement here and there on corei7-level machines, but the actual results depend a lot on the used microarchitecture. The transformation is currently disabled by default and available by passing the -enable-cgp-select2branch flag to the code generator. Thanks to Chandler for the initial test case to him and Evan Cheng for providing me with comments and test-suite numbers that were more stable than mine :) llvm-svn: 156234
2012-05-05 20:49:22 +08:00
%cmp = fcmp ogt double %a, %b
%cond = select i1 %cmp, i32 %x, i32 %y
ret i32 %cond
}
; Multiple uses of the load.
define i32 @test4(i32 %a, i32* nocapture %b, i32 %x, i32 %y) {
[x86] auto-generate checks for cmov tests llvm-svn: 267417
2016-04-25 23:26:57 +08:00
; CHECK-LABEL: test4:
; CHECK: # BB#0:
; CHECK-NEXT: movl (%rsi), %eax
; CHECK-NEXT: cmpl %edi, %eax
; CHECK-NEXT: cmovael %ecx, %edx
; CHECK-NEXT: addl %edx, %eax
; CHECK-NEXT: retq
;
[opaque pointer type] Add textual IR support for explicit type parameter to load instruction Essentially the same as the GEP change in r230786. A similar migration script can be used to update test cases, though a few more test case improvements/changes were required this time around: (r229269-r229278) import fileinput import sys import re pat = re.compile(r"((?:=|:|^)\s*load (?:atomic )?(?:volatile )?(.*?))(| addrspace\(\d+\) *)\*($| *(?:%|@|null|undef|blockaddress|getelementptr|addrspacecast|bitcast|inttoptr|\[\[[a-zA-Z]|\{\{).*$)") for line in sys.stdin: sys.stdout.write(re.sub(pat, r"\1, \2\3*\4", line)) Reviewers: rafael, dexonsmith, grosser Differential Revision: http://reviews.llvm.org/D7649 llvm-svn: 230794
2015-02-28 05:17:42 +08:00
%load = load i32, i32* %b, align 4
CodeGenPrepare: Add a transform to turn selects into branches in some cases. This came up when a change in block placement formed a cmov and slowed down a hot loop by 50%: ucomisd (%rdi), %xmm0 cmovbel %edx, %esi cmov is a really bad choice in this context because it doesn't get branch prediction. If we emit it as a branch, an out-of-order CPU can do a better job (if the branch is predicted right) and avoid waiting for the slow load+compare instruction to finish. Of course it won't help if the branch is unpredictable, but those are really rare in practice. This patch uses a dumb conservative heuristic, it turns all cmovs that have one use and a direct memory operand into branches. cmovs usually save some code size, so we disable the transform in -Os mode. In-Order architectures are unlikely to benefit as well, those are included in the "predictableSelectIsExpensive" flag. It would be better to reuse branch probability info here, but BPI doesn't support select instructions currently. It would make sense to use the same heuristics as the if-converter pass, which does the opposite direction of this transform. Test suite shows a small improvement here and there on corei7-level machines, but the actual results depend a lot on the used microarchitecture. The transformation is currently disabled by default and available by passing the -enable-cgp-select2branch flag to the code generator. Thanks to Chandler for the initial test case to him and Evan Cheng for providing me with comments and test-suite numbers that were more stable than mine :) llvm-svn: 156234
2012-05-05 20:49:22 +08:00
%cmp = icmp ult i32 %load, %a
%cond = select i1 %cmp, i32 %x, i32 %y
%add = add i32 %cond, %load
ret i32 %add
}
; Multiple uses of the cmp.
define i32 @test5(i32 %a, i32* nocapture %b, i32 %x, i32 %y) {
[x86] auto-generate checks for cmov tests llvm-svn: 267417
2016-04-25 23:26:57 +08:00
; CHECK-LABEL: test5:
; CHECK: # BB#0:
; CHECK-NEXT: cmpl %edi, (%rsi)
; CHECK-NEXT: cmoval %edi, %ecx
; CHECK-NEXT: cmovael %edx, %ecx
; CHECK-NEXT: movl %ecx, %eax
; CHECK-NEXT: retq
;
[opaque pointer type] Add textual IR support for explicit type parameter to load instruction Essentially the same as the GEP change in r230786. A similar migration script can be used to update test cases, though a few more test case improvements/changes were required this time around: (r229269-r229278) import fileinput import sys import re pat = re.compile(r"((?:=|:|^)\s*load (?:atomic )?(?:volatile )?(.*?))(| addrspace\(\d+\) *)\*($| *(?:%|@|null|undef|blockaddress|getelementptr|addrspacecast|bitcast|inttoptr|\[\[[a-zA-Z]|\{\{).*$)") for line in sys.stdin: sys.stdout.write(re.sub(pat, r"\1, \2\3*\4", line)) Reviewers: rafael, dexonsmith, grosser Differential Revision: http://reviews.llvm.org/D7649 llvm-svn: 230794
2015-02-28 05:17:42 +08:00
%load = load i32, i32* %b, align 4
CodeGenPrepare: Add a transform to turn selects into branches in some cases. This came up when a change in block placement formed a cmov and slowed down a hot loop by 50%: ucomisd (%rdi), %xmm0 cmovbel %edx, %esi cmov is a really bad choice in this context because it doesn't get branch prediction. If we emit it as a branch, an out-of-order CPU can do a better job (if the branch is predicted right) and avoid waiting for the slow load+compare instruction to finish. Of course it won't help if the branch is unpredictable, but those are really rare in practice. This patch uses a dumb conservative heuristic, it turns all cmovs that have one use and a direct memory operand into branches. cmovs usually save some code size, so we disable the transform in -Os mode. In-Order architectures are unlikely to benefit as well, those are included in the "predictableSelectIsExpensive" flag. It would be better to reuse branch probability info here, but BPI doesn't support select instructions currently. It would make sense to use the same heuristics as the if-converter pass, which does the opposite direction of this transform. Test suite shows a small improvement here and there on corei7-level machines, but the actual results depend a lot on the used microarchitecture. The transformation is currently disabled by default and available by passing the -enable-cgp-select2branch flag to the code generator. Thanks to Chandler for the initial test case to him and Evan Cheng for providing me with comments and test-suite numbers that were more stable than mine :) llvm-svn: 156234
2012-05-05 20:49:22 +08:00
%cmp = icmp ult i32 %load, %a
%cmp1 = icmp ugt i32 %load, %a
%cond = select i1 %cmp1, i32 %a, i32 %y
%cond5 = select i1 %cmp, i32 %cond, i32 %x
ret i32 %cond5
}
add tests for potential CGP transform (PR27344) llvm-svn: 267426
2016-04-26 00:56:52 +08:00
; If a select is not obviously predictable, don't turn it into a branch.
define i32 @weighted_select1(i32 %a, i32 %b) {
; CHECK-LABEL: weighted_select1:
; CHECK: # BB#0:
; CHECK-NEXT: testl %edi, %edi
; CHECK-NEXT: cmovnel %edi, %esi
; CHECK-NEXT: movl %esi, %eax
; CHECK-NEXT: retq
;
%cmp = icmp ne i32 %a, 0
%sel = select i1 %cmp, i32 %a, i32 %b, !prof !0
ret i32 %sel
}
[CodeGenPrepare] use branch weight metadata to decide if a select should be turned into a branch This is part of solving PR27344: https://llvm.org/bugs/show_bug.cgi?id=27344 CGP should undo the SimplifyCFG transform for the same reason that earlier patches have used this same mechanism: it's possible that passes between SimplifyCFG and CGP may be able to optimize the IR further with a select in place. For the TLI hook default, >99% taken or not taken is chosen as the default threshold for a highly predictable branch. Even the most limited HW branch predictors will be correct on this branch almost all the time, so even a massive mispredict penalty perf loss would be overcome by the win from all the times the branch was predicted correctly. As a follow-up, we could make the default target hook less conservative by using the SchedMachineModel's MispredictPenalty. Or we could just let targets override the default by implementing the hook with that and other target-specific options. Note that trying to statically determine mispredict rates for close-to-balanced profile weight data is generally impossible if the HW is sufficiently advanced. Ie, 50/50 taken/not-taken might still be 100% predictable. Finally, note that this patch as-is will not solve PR27344 because the current __builtin_unpredictable() branch weight default values are 4 and 64. A proposal to change that is in D19435. Differential Revision: http://reviews.llvm.org/D19488 llvm-svn: 267572
2016-04-27 01:11:17 +08:00
; If a select is obviously predictable, turn it into a branch.
add tests for potential CGP transform (PR27344) llvm-svn: 267426
2016-04-26 00:56:52 +08:00
define i32 @weighted_select2(i32 %a, i32 %b) {
; CHECK-LABEL: weighted_select2:
; CHECK: # BB#0:
; CHECK-NEXT: testl %edi, %edi
[CodeGenPrepare] use branch weight metadata to decide if a select should be turned into a branch This is part of solving PR27344: https://llvm.org/bugs/show_bug.cgi?id=27344 CGP should undo the SimplifyCFG transform for the same reason that earlier patches have used this same mechanism: it's possible that passes between SimplifyCFG and CGP may be able to optimize the IR further with a select in place. For the TLI hook default, >99% taken or not taken is chosen as the default threshold for a highly predictable branch. Even the most limited HW branch predictors will be correct on this branch almost all the time, so even a massive mispredict penalty perf loss would be overcome by the win from all the times the branch was predicted correctly. As a follow-up, we could make the default target hook less conservative by using the SchedMachineModel's MispredictPenalty. Or we could just let targets override the default by implementing the hook with that and other target-specific options. Note that trying to statically determine mispredict rates for close-to-balanced profile weight data is generally impossible if the HW is sufficiently advanced. Ie, 50/50 taken/not-taken might still be 100% predictable. Finally, note that this patch as-is will not solve PR27344 because the current __builtin_unpredictable() branch weight default values are 4 and 64. A proposal to change that is in D19435. Differential Revision: http://reviews.llvm.org/D19488 llvm-svn: 267572
2016-04-27 01:11:17 +08:00
; CHECK-NEXT: jne [[LABEL_BB5:.*]]
; CHECK: movl %esi, %edi
; CHECK-NEXT: [[LABEL_BB5]]
; CHECK-NEXT: movl %edi, %eax
add tests for potential CGP transform (PR27344) llvm-svn: 267426
2016-04-26 00:56:52 +08:00
; CHECK-NEXT: retq
;
%cmp = icmp ne i32 %a, 0
%sel = select i1 %cmp, i32 %a, i32 %b, !prof !1
ret i32 %sel
}
[CodeGenPrepare] use branch weight metadata to decide if a select should be turned into a branch This is part of solving PR27344: https://llvm.org/bugs/show_bug.cgi?id=27344 CGP should undo the SimplifyCFG transform for the same reason that earlier patches have used this same mechanism: it's possible that passes between SimplifyCFG and CGP may be able to optimize the IR further with a select in place. For the TLI hook default, >99% taken or not taken is chosen as the default threshold for a highly predictable branch. Even the most limited HW branch predictors will be correct on this branch almost all the time, so even a massive mispredict penalty perf loss would be overcome by the win from all the times the branch was predicted correctly. As a follow-up, we could make the default target hook less conservative by using the SchedMachineModel's MispredictPenalty. Or we could just let targets override the default by implementing the hook with that and other target-specific options. Note that trying to statically determine mispredict rates for close-to-balanced profile weight data is generally impossible if the HW is sufficiently advanced. Ie, 50/50 taken/not-taken might still be 100% predictable. Finally, note that this patch as-is will not solve PR27344 because the current __builtin_unpredictable() branch weight default values are 4 and 64. A proposal to change that is in D19435. Differential Revision: http://reviews.llvm.org/D19488 llvm-svn: 267572
2016-04-27 01:11:17 +08:00
; Note the reversed profile weights: it doesn't matter if it's
; obviously true or obviously false.
; Either one should become a branch rather than conditional move.
; TODO: But likely true vs. likely false should affect basic block placement?
define i32 @weighted_select3(i32 %a, i32 %b) {
; CHECK-LABEL: weighted_select3:
; CHECK: # BB#0:
; CHECK-NEXT: testl %edi, %edi
[Profile] preserve branch metadata lowering select in CGP CGP currently drops select's MD_prof profile data when generating conditional branch which can lead to bad code layout. The patch fixes the issue. Differential Revision: http://reviews.llvm.org/D24169 llvm-svn: 280600
2016-09-04 05:26:36 +08:00
; CHECK-NEXT: je [[LABEL_BB6:.*]]
; CHECK: movl %edi, %eax
Codegen: Tail-duplicate during placement. The tail duplication pass uses an assumed layout when making duplication decisions. This is fine, but passes up duplication opportunities that may arise when blocks are outlined. Because we want the updated CFG to affect subsequent placement decisions, this change must occur during placement. In order to achieve this goal, TailDuplicationPass is split into a utility class, TailDuplicator, and the pass itself. The pass delegates nearly everything to the TailDuplicator object, except for looping over the blocks in a function. This allows the same code to be used for tail duplication in both places. This change, in concert with outlining optional branches, allows triangle shaped code to perform much better, esepecially when the taken/untaken branches are correlated, as it creates a second spine when the tests are small enough. Issue from previous rollback fixed, and a new test was added for that case as well. Issue was worklist/scheduling/taildup issue in layout. Issue from 2nd rollback fixed, with 2 additional tests. Issue was tail merging/loop info/tail-duplication causing issue with loops that share a header block. Issue with early tail-duplication of blocks that branch to a fallthrough predecessor fixed with test case: tail-dup-branch-to-fallthrough.ll Differential revision: https://reviews.llvm.org/D18226 llvm-svn: 283934
2016-10-12 04:36:43 +08:00
; CHECK-NEXT: retq
[Profile] preserve branch metadata lowering select in CGP CGP currently drops select's MD_prof profile data when generating conditional branch which can lead to bad code layout. The patch fixes the issue. Differential Revision: http://reviews.llvm.org/D24169 llvm-svn: 280600
2016-09-04 05:26:36 +08:00
; CHECK: [[LABEL_BB6]]
; CHECK-NEXT: movl %esi, %edi
Codegen: Tail-duplicate during placement. The tail duplication pass uses an assumed layout when making duplication decisions. This is fine, but passes up duplication opportunities that may arise when blocks are outlined. Because we want the updated CFG to affect subsequent placement decisions, this change must occur during placement. In order to achieve this goal, TailDuplicationPass is split into a utility class, TailDuplicator, and the pass itself. The pass delegates nearly everything to the TailDuplicator object, except for looping over the blocks in a function. This allows the same code to be used for tail duplication in both places. This change, in concert with outlining optional branches, allows triangle shaped code to perform much better, esepecially when the taken/untaken branches are correlated, as it creates a second spine when the tests are small enough. Issue from previous rollback fixed, and a new test was added for that case as well. Issue was worklist/scheduling/taildup issue in layout. Issue from 2nd rollback fixed, with 2 additional tests. Issue was tail merging/loop info/tail-duplication causing issue with loops that share a header block. Issue with early tail-duplication of blocks that branch to a fallthrough predecessor fixed with test case: tail-dup-branch-to-fallthrough.ll Differential revision: https://reviews.llvm.org/D18226 llvm-svn: 283934
2016-10-12 04:36:43 +08:00
; CHECK-NEXT: movl %edi, %eax
; CHECK-NEXT: retq
[CodeGenPrepare] use branch weight metadata to decide if a select should be turned into a branch This is part of solving PR27344: https://llvm.org/bugs/show_bug.cgi?id=27344 CGP should undo the SimplifyCFG transform for the same reason that earlier patches have used this same mechanism: it's possible that passes between SimplifyCFG and CGP may be able to optimize the IR further with a select in place. For the TLI hook default, >99% taken or not taken is chosen as the default threshold for a highly predictable branch. Even the most limited HW branch predictors will be correct on this branch almost all the time, so even a massive mispredict penalty perf loss would be overcome by the win from all the times the branch was predicted correctly. As a follow-up, we could make the default target hook less conservative by using the SchedMachineModel's MispredictPenalty. Or we could just let targets override the default by implementing the hook with that and other target-specific options. Note that trying to statically determine mispredict rates for close-to-balanced profile weight data is generally impossible if the HW is sufficiently advanced. Ie, 50/50 taken/not-taken might still be 100% predictable. Finally, note that this patch as-is will not solve PR27344 because the current __builtin_unpredictable() branch weight default values are 4 and 64. A proposal to change that is in D19435. Differential Revision: http://reviews.llvm.org/D19488 llvm-svn: 267572
2016-04-27 01:11:17 +08:00
;
%cmp = icmp ne i32 %a, 0
%sel = select i1 %cmp, i32 %a, i32 %b, !prof !2
ret i32 %sel
}
[CGP] avoid crashing from weightlessness It's possible that we have branch weights with 0 values. In that case, don't try to create an impossible BranchProbability. llvm-svn: 268935
2016-05-10 01:31:55 +08:00
; Weightlessness is no reason to die.
define i32 @unweighted_select(i32 %a, i32 %b) {
; CHECK-LABEL: unweighted_select:
; CHECK: # BB#0:
; CHECK-NEXT: testl %edi, %edi
; CHECK-NEXT: cmovnel %edi, %esi
; CHECK-NEXT: movl %esi, %eax
; CHECK-NEXT: retq
;
%cmp = icmp ne i32 %a, 0
%sel = select i1 %cmp, i32 %a, i32 %b, !prof !3
ret i32 %sel
}
[CodeGenPrepare] use branch weight metadata to decide if a select should be turned into a branch This is part of solving PR27344: https://llvm.org/bugs/show_bug.cgi?id=27344 CGP should undo the SimplifyCFG transform for the same reason that earlier patches have used this same mechanism: it's possible that passes between SimplifyCFG and CGP may be able to optimize the IR further with a select in place. For the TLI hook default, >99% taken or not taken is chosen as the default threshold for a highly predictable branch. Even the most limited HW branch predictors will be correct on this branch almost all the time, so even a massive mispredict penalty perf loss would be overcome by the win from all the times the branch was predicted correctly. As a follow-up, we could make the default target hook less conservative by using the SchedMachineModel's MispredictPenalty. Or we could just let targets override the default by implementing the hook with that and other target-specific options. Note that trying to statically determine mispredict rates for close-to-balanced profile weight data is generally impossible if the HW is sufficiently advanced. Ie, 50/50 taken/not-taken might still be 100% predictable. Finally, note that this patch as-is will not solve PR27344 because the current __builtin_unpredictable() branch weight default values are 4 and 64. A proposal to change that is in D19435. Differential Revision: http://reviews.llvm.org/D19488 llvm-svn: 267572
2016-04-27 01:11:17 +08:00
add tests for potential CGP transform (PR27344) llvm-svn: 267426
2016-04-26 00:56:52 +08:00
!0 = !{!"branch_weights", i32 1, i32 99}
!1 = !{!"branch_weights", i32 1, i32 100}
[CodeGenPrepare] use branch weight metadata to decide if a select should be turned into a branch This is part of solving PR27344: https://llvm.org/bugs/show_bug.cgi?id=27344 CGP should undo the SimplifyCFG transform for the same reason that earlier patches have used this same mechanism: it's possible that passes between SimplifyCFG and CGP may be able to optimize the IR further with a select in place. For the TLI hook default, >99% taken or not taken is chosen as the default threshold for a highly predictable branch. Even the most limited HW branch predictors will be correct on this branch almost all the time, so even a massive mispredict penalty perf loss would be overcome by the win from all the times the branch was predicted correctly. As a follow-up, we could make the default target hook less conservative by using the SchedMachineModel's MispredictPenalty. Or we could just let targets override the default by implementing the hook with that and other target-specific options. Note that trying to statically determine mispredict rates for close-to-balanced profile weight data is generally impossible if the HW is sufficiently advanced. Ie, 50/50 taken/not-taken might still be 100% predictable. Finally, note that this patch as-is will not solve PR27344 because the current __builtin_unpredictable() branch weight default values are 4 and 64. A proposal to change that is in D19435. Differential Revision: http://reviews.llvm.org/D19488 llvm-svn: 267572
2016-04-27 01:11:17 +08:00
!2 = !{!"branch_weights", i32 100, i32 1}
[CGP] avoid crashing from weightlessness It's possible that we have branch weights with 0 values. In that case, don't try to create an impossible BranchProbability. llvm-svn: 268935
2016-05-10 01:31:55 +08:00
!3 = !{!"branch_weights", i32 0, i32 0}
add tests for potential CGP transform (PR27344) llvm-svn: 267426
2016-04-26 00:56:52 +08:00