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[X86] Disable nop padding before instruction following a prefix

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Reviewers: reames, MaskRay, craig.topper, LuoYuanke, jyknight

Reviewed By: LuoYuanke

Subscribers: hiraditya, llvm-commits, annita.zhang

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76052
This commit is contained in:
Shengchen Kan 2020-03-12 17:13:09 +08:00
parent f65b040dae
commit 4b036bee4a
2 changed files with 86 additions and 0 deletions
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10
lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
View File

@ -332,6 +332,11 @@ static bool isRIPRelative(const MCInst &MI, const MCInstrInfo &MCII) {
return (BaseReg == X86::RIP);
}
/// Check if the instruction is a prefix.
static bool isPrefix(const MCInst &MI, const MCInstrInfo &MCII) {
return X86II::isPrefix(MCII.get(MI.getOpcode()).TSFlags);
}
/// Check if the instruction is valid as the first instruction in macro fusion.
static bool isFirstMacroFusibleInst(const MCInst &Inst,
const MCInstrInfo &MCII) {
@ -505,6 +510,11 @@ void X86AsmBackend::alignBranchesBegin(MCObjectStreamer &OS,
// instruction delay, inserting a nop would change behavior.
return;
if (isPrefix(PrevInst, *MCII))
// If this instruction follows a prefix, inserting a nop would change
// semantic.
return;
if (!isMacroFused(PrevInst, Inst))
// Macro fusion doesn't happen indeed, clear the pending.
PendingBoundaryAlign = nullptr;

76
test/MC/X86/align-branch-64-prefix.s Normal file
View File

@ -0,0 +1,76 @@
# RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu --x86-align-branch-boundary=32 --x86-align-branch=jmp+call %s | llvm-objdump -d --no-show-raw-insn - | FileCheck %s
# Exercise cases where prefixes are specified for instructions to be aligned
# and thus can't add a nop in between without changing semantic.
.text
# CHECK: 1d: int3
# CHECK: 1e: jmp
# CHECK: 24: int3
.p2align 5
.rept 30
int3
.endr
CS
jmp baz
int3
# CHECK: 5d: int3
# CHECK: 5e: jmp
# CHECK: 64: int3
.p2align 5
.rept 30
int3
.endr
GS
jmp baz
int3
# CHECK: 9d: int3
# CHECK: 9e: call
# CHECK: a6: int3
.p2align 5
.rept 30
int3
.endr
data16
call *___tls_get_addr@GOT(%ecx)
int3
# CHECK: de: lock
# CHECK: df: jmp
# CHECK: e4: int3
.p2align 5
.rept 30
int3
.endr
lock
jmp baz
int3
# CHECK: 11d: int3
# CHECK: 11e: jmp
# CHECK: 124: int3
.p2align 5
.rept 30
int3
.endr
rex64
jmp baz
int3
# CHECK: 15d: int3
# CHECK: 15e: {{.*}} jmp
# CHECK: 164: int3
.p2align 5
.rept 30
int3
.endr
xacquire
jmp baz
int3
.section ".text.other"
bar:
retq