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allow branch weight metadata on select instructions (PR26636) 

As noted in:
https://llvm.org/bugs/show_bug.cgi?id=26636

This doesn't accomplish anything on its own. It's the first step towards preserving 
and using branch weights with selects.

The next step would be to make sure we're propagating the info in all of the other
places where we create selects (SimplifyCFG, InstCombine, etc). I don't think there's
an easy fix to make this happen; we have to look at each transform individually to 
determine how to correctly propagate the weights.

Along with that step, we need to then use the weights when making subsequent transform
decisions such as discussed in http://reviews.llvm.org/D16836.

The inliner test is independent but closely related. It verifies that metadata is
preserved when both branches and selects are cloned.

Differential Revision: http://reviews.llvm.org/D18133

llvm-svn: 263482
This commit is contained in:
Sanjay Patel 2016-03-14 20:18:59 +00:00
parent 9d94397859
commit ee52b6e77d
4 changed files with 58 additions and 5 deletions
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8
llvm/include/llvm/IR/IRBuilder.h
View File

@ -1569,12 +1569,16 @@ public:
}
Value *CreateSelect(Value *C, Value *True, Value *False,
const Twine &Name = "") {
const Twine &Name = "", MDNode *ProfWeights = nullptr) {
if (Constant *CC = dyn_cast<Constant>(C))
if (Constant *TC = dyn_cast<Constant>(True))
if (Constant *FC = dyn_cast<Constant>(False))
return Insert(Folder.CreateSelect(CC, TC, FC), Name);
return Insert(SelectInst::Create(C, True, False), Name);
SelectInst *Sel = SelectInst::Create(C, True, False);
// TODO: "unpredictable" metadata can apply to a select too.
Sel->setMetadata(LLVMContext::MD_prof, ProfWeights);
return Insert(Sel, Name);
}
VAArgInst *CreateVAArg(Value *List, Type *Ty, const Twine &Name = "") {

3
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -1935,7 +1935,8 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
Value *Select = Builder.CreateSelect(IfCond, TrueVal, FalseVal);
MDNode *MDN = InsertPt->getMetadata(LLVMContext::MD_prof);
Value *Select = Builder.CreateSelect(IfCond, TrueVal, FalseVal, "", MDN);
PN->replaceAllUsesWith(Select);
Select->takeName(PN);
PN->eraseFromParent();

41
llvm/test/Transforms/Inline/profile-meta.ll Normal file
View File

@ -0,0 +1,41 @@
; RUN: opt < %s -S -inline | FileCheck %s
; Make sure that profile metadata is preserved when cloning a select.
define i32 @callee_with_select(i1 %c, i32 %a, i32 %b) {
%sel = select i1 %c, i32 %a, i32 %b, !prof !0
ret i32 %sel
}
define i32 @caller_of_select(i1 %C, i32 %A, i32 %B) {
%ret = call i32 @callee_with_select(i1 %C, i32 %A, i32 %B)
ret i32 %ret
; CHECK-LABEL: @caller_of_select(
; CHECK-NEXT: [[SEL:%.*]] = select i1 %C, i32 %A, i32 %B, !prof !0
; CHECK-NEXT: ret i32 [[SEL]]
}
; Make sure that profile metadata is preserved when cloning a branch.
define i32 @callee_with_branch(i1 %c) {
br i1 %c, label %if, label %else, !prof !1
if:
ret i32 1
else:
ret i32 2
}
define i32 @caller_of_branch(i1 %C) {
%ret = call i32 @callee_with_branch(i1 %C)
ret i32 %ret
; CHECK-LABEL: @caller_of_branch(
; CHECK-NEXT: br i1 %C, label %{{.*}}, label %{{.*}}, !prof !1
}
!0 = !{!"branch_weights", i32 1, i32 2}
!1 = !{!"branch_weights", i32 3, i32 4}
; CHECK: !0 = !{!"branch_weights", i32 1, i32 2}
; CHECK: !1 = !{!"branch_weights", i32 3, i32 4}

11
llvm/test/Transforms/SimplifyCFG/PhiEliminate2.ll
View File

@ -1,8 +1,11 @@
; RUN: opt < %s -simplifycfg -S | FileCheck %s
; Use a select to make this a single BB.
; Also, make sure the profile metadata is propagated to the select (PR26636).
define i32 @FoldTwoEntryPHINode(i1 %C, i32 %V1, i32 %V2, i16 %V3) {
entry:
br i1 %C, label %then, label %else
br i1 %C, label %then, label %else, !prof !0
then:
%V4 = or i32 %V2, %V1
br label %Cont
@ -18,8 +21,12 @@ Cont:
; CHECK-NEXT: entry:
; CHECK-NEXT: %V5 = sext i16 %V3 to i32
; CHECK-NEXT: %V4 = or i32 %V2, %V1
; CHECK-NEXT: %V6 = select i1 %C, i32 %V4, i32 %V5
; CHECK-NEXT: %V6 = select i1 %C, i32 %V4, i32 %V5, !prof !0
; CHECK-NEXT: %0 = call i32 @FoldTwoEntryPHINode(i1 false, i32 0, i32 0, i16 0)
; CHECK-NEXT: ret i32 %V1
}
!0 = !{!"branch_weights", i32 3, i32 5}
; CHECK: !0 = !{!"branch_weights", i32 3, i32 5}