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Use 15 byte long nops on modern Intel processors 

Back in D42616, we switched our default nop length from 15 to 10 bytes because some platforms have painful decode stalls when encountering multiple instruction prefixes. (10 byte long nops come from the fact that prefixes are used to pad after 8 bytes, and some platforms have issues w/more than two prefixes.)

Based on Agner's guides, it appears to be the case that modern Intel (SandyBridge and later) can decode an arbitrary number of prefixes without issue. Intel's guide only provides up to 9 bytes; I read that as providing a safe default for all their chips. Older chips and Atom series have serious decode stalls. I can't find a conclusive reference beyond those two.

Differential Revision: https://reviews.llvm.org/D75945
This commit is contained in:
Philip Reames 2020-03-13 10:49:38 -07:00
parent a26bd4ec16
commit 1b86ad27a7
4 changed files with 16 additions and 12 deletions
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3
llvm/lib/Target/X86/X86.td
View File

@ -551,7 +551,8 @@ def ProcessorFeatures {
FeatureSlow3OpsLEA, FeatureSlow3OpsLEA,
FeatureFastScalarFSQRT, FeatureFastScalarFSQRT,
FeatureFastSHLDRotate, FeatureFastSHLDRotate,
FeatureMergeToThreeWayBranch]; FeatureMergeToThreeWayBranch,
FeatureFast15ByteNOP];
list<SubtargetFeature> SNBSpecificFeatures = [FeatureSlowUAMem32, list<SubtargetFeature> SNBSpecificFeatures = [FeatureSlowUAMem32,
FeaturePOPCNTFalseDeps]; FeaturePOPCNTFalseDeps];
list<SubtargetFeature> SNBInheritableFeatures = list<SubtargetFeature> SNBInheritableFeatures =

6
llvm/test/CodeGen/X86/align-branch-boundary-suppressions.ll
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@ -69,8 +69,12 @@ define void @patchpoint(i64 %a, i64 %b) {
; CHECK-NEXT: .cfi_def_cfa_register %rbp ; CHECK-NEXT: .cfi_def_cfa_register %rbp
; CHECK-NEXT: #noautopadding ; CHECK-NEXT: #noautopadding
; CHECK-NEXT: .Ltmp2: ; CHECK-NEXT: .Ltmp2:
; CHECK-NEXT: .byte 102
; CHECK-NEXT: .byte 102
; CHECK-NEXT: .byte 102
; CHECK-NEXT: .byte 102
; CHECK-NEXT: .byte 102
; CHECK-NEXT: nopw %cs:512(%rax,%rax) ; CHECK-NEXT: nopw %cs:512(%rax,%rax)
; CHECK-NEXT: nopl 8(%rax,%rax)
; CHECK-NEXT: #autopadding ; CHECK-NEXT: #autopadding
; CHECK-NEXT: popq %rbp ; CHECK-NEXT: popq %rbp
; CHECK-NEXT: .cfi_def_cfa %rsp, 8 ; CHECK-NEXT: .cfi_def_cfa %rsp, 8

7
llvm/test/MC/X86/align-via-relaxation.s
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@ -32,8 +32,7 @@ foo:
# that would require a further round of relaxation # that would require a further round of relaxation
# CHECK: <bar>: # CHECK: <bar>:
# CHECK: 22: eb fe jmp -2 <bar> # CHECK: 22: eb fe jmp -2 <bar>
# CHECK: 24: 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:(%rax,%rax) # CHECK: 24: 66 66 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:(%rax,%rax)
# CHECK: 2e: 66 90 nop
# CHECK: 30: 0f 0b ud2 # CHECK: 30: 0f 0b ud2
bar: bar:
@ -48,8 +47,8 @@ nobypass:
# CHECK: <loop_preheader>: # CHECK: <loop_preheader>:
# CHECK: 45: 48 85 c0 testq %rax, %rax # CHECK: 45: 48 85 c0 testq %rax, %rax
# CHECK: 48: 0f 8e 22 00 00 00 jle 34 <loop_exit> # CHECK: 48: 0f 8e 22 00 00 00 jle 34 <loop_exit>
# CHECK: 4e: 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:(%rax,%rax) # CHECK: 4e: 66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:(%rax,%rax)
# CHECK: 58: 0f 1f 84 00 00 00 00 00 nopl (%rax,%rax) # CHECK: 5d: 0f 1f 00 nopl (%rax)
# CHECK: <loop_header>: # CHECK: <loop_header>:
# CHECK: 60: 48 83 e8 01 subq $1, %rax # CHECK: 60: 48 83 e8 01 subq $1, %rax
# CHECK: 64: 48 85 c0 testq %rax, %rax # CHECK: 64: 48 85 c0 testq %rax, %rax

12
llvm/test/MC/X86/x86_long_nop.s
View File

@ -17,12 +17,12 @@
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu %s -mcpu=znver2 | llvm-objdump -d -no-show-raw-insn - | FileCheck %s --check-prefix=LNOP15 # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu %s -mcpu=znver2 | llvm-objdump -d -no-show-raw-insn - | FileCheck %s --check-prefix=LNOP15
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=nehalem %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=nehalem %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=westmere %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=westmere %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=sandybridge %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=sandybridge %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP15 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=ivybridge %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=ivybridge %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP15 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=haswell %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=haswell %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP15 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=broadwell %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=broadwell %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP15 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=skylake %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=skylake %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP15 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=skx %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=skx %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP15 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=knl %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=knl %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s
# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=knm %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=knm %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP10 %s