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[X86][APX] Use TEST instruction for CLOAD/CSTORE #151160

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Merged
phoebewang merged 1 commit into from
Jul 30, 2025
Merged

[X86][APX] Use TEST instruction for CLOAD/CSTORE #151160

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6 changes: 2 additions & 4 deletions llvm/lib/Target/X86/X86ISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -58074,11 +58074,9 @@ static SDValue combineX86CloadCstore(SDNode *N, SelectionDAG &DAG) {
// res, flags2 = sub 0, (and X, Y)
// cload/cstore ..., cond_ne, flag2
// ->
// res, flags2 = and X, Y
// res, flags2 = cmp (and X, Y), 0
// cload/cstore ..., cond_ne, flag2
Ops[4] = DAG.getNode(X86ISD::AND, DL, Sub->getVTList(), Op1.getOperand(0),
Op1.getOperand(1))
.getValue(1);
Ops[4] = DAG.getNode(X86ISD::CMP, DL, MVT::i32, Op1, Sub.getOperand(0));
} else {
return SDValue();
}
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4 changes: 2 additions & 2 deletions llvm/test/CodeGen/X86/apx/cf.ll
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Expand Up @@ -162,7 +162,7 @@ entry:
define void @load_zext(i1 %cond, ptr %b, ptr %p) {
; CHECK-LABEL: load_zext:
; CHECK: # %bb.0: # %entry
; CHECK-NEXT: andb $1, %dil
; CHECK-NEXT: testb $1, %dil
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@KanRobert KanRobert Jul 29, 2025

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Why it's not optimized to test by eflags-related peephole optimization?

I remember we used and instead of test to give more chance for CSE.

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@phoebewang phoebewang Jul 30, 2025

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I only see cmp <-> test conversions. I think test is always preferred when only eflags of an and is used, hence pattern:

def X86testpat : PatFrag<(ops node:$lhs, node:$rhs),
                         (X86cmp (and_su node:$lhs, node:$rhs), 0)>;

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@KanRobert KanRobert Jul 30, 2025

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@phoebewang Then it seems clang generates the worse code than gcc about this.

https://www.godbolt.org/z/T7djcxv5e

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@phoebewang phoebewang Jul 30, 2025

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@phoebewang Then it seems clang generates the worse code than gcc about this.

https://www.godbolt.org/z/T7djcxv5e

Why worse? It reduces a branch.

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@KanRobert KanRobert Jul 30, 2025

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My fault. It's an incorrect example.

; CHECK-NEXT: cfcmovnew (%rsi), %ax
; CHECK-NEXT: movzwl %ax, %eax
; CHECK-NEXT: cfcmovnel %eax, (%rdx)
Expand All @@ -180,7 +180,7 @@ entry:
define void @load_sext(i1 %cond, ptr %b, ptr %p) {
; CHECK-LABEL: load_sext:
; CHECK: # %bb.0: # %entry
; CHECK-NEXT: andb $1, %dil
; CHECK-NEXT: testb $1, %dil
; CHECK-NEXT: cfcmovnel (%rsi), %eax
; CHECK-NEXT: cltq
; CHECK-NEXT: cfcmovneq %rax, (%rdx)
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