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[InstCombine] Fix infinite loop in select equivalence fold #84036

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Mar 6, 2024
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[InstCombine] Fix infinite loop in select equivalence fold #84036

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9 changes: 7 additions & 2 deletions llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1285,7 +1285,11 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
isGuaranteedNotToBeUndefOrPoison(CmpRHS, SQ.AC, &Sel, &DT)) {
if (Value *V = simplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, SQ,
/* AllowRefinement */ true))
return replaceOperand(Sel, Swapped ? 2 : 1, V);
// Require either the replacement or the simplification result to be a
// constant to avoid infinite loops.
// FIXME: Make this check more precise.
if (isa<Constant>(CmpRHS) || isa<Constant>(V))
return replaceOperand(Sel, Swapped ? 2 : 1, V);

// Even if TrueVal does not simplify, we can directly replace a use of
// CmpLHS with CmpRHS, as long as the instruction is not used anywhere
Expand All @@ -1303,7 +1307,8 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
isGuaranteedNotToBeUndefOrPoison(CmpLHS, SQ.AC, &Sel, &DT))
if (Value *V = simplifyWithOpReplaced(TrueVal, CmpRHS, CmpLHS, SQ,
/* AllowRefinement */ true))
return replaceOperand(Sel, Swapped ? 2 : 1, V);
if (isa<Constant>(CmpLHS) || isa<Constant>(V))
return replaceOperand(Sel, Swapped ? 2 : 1, V);

auto *FalseInst = dyn_cast<Instruction>(FalseVal);
if (!FalseInst)
Expand Down
38 changes: 37 additions & 1 deletion llvm/test/Transforms/InstCombine/select.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -2849,12 +2849,14 @@ define i8 @select_replacement_sub(i8 %x, i8 %y, i8 %z) {
ret i8 %sel
}

; FIXME: This is safe to fold.
define i8 @select_replacement_shift_noundef(i8 %x, i8 %y, i8 %z) {
; CHECK-LABEL: @select_replacement_shift_noundef(
; CHECK-NEXT: [[SHR:%.*]] = lshr exact i8 [[X:%.*]], 1
; CHECK-NEXT: call void @use_i8(i8 noundef [[SHR]])
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[SHR]], [[Y:%.*]]
; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i8 [[X]], i8 [[Z:%.*]]
; CHECK-NEXT: [[SHL:%.*]] = shl i8 [[Y]], 1
; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i8 [[SHL]], i8 [[Z:%.*]]
; CHECK-NEXT: ret i8 [[SEL]]
;
%shr = lshr exact i8 %x, 1
Expand Down Expand Up @@ -2904,6 +2906,40 @@ define i32 @select_replacement_loop2(i32 %arg, i32 %arg2) {
ret i32 %sel
}

define i8 @select_replacement_loop3(i32 noundef %x) {
; CHECK-LABEL: @select_replacement_loop3(
; CHECK-NEXT: [[TRUNC:%.*]] = trunc i32 [[X:%.*]] to i8
; CHECK-NEXT: [[REV:%.*]] = call i8 @llvm.bitreverse.i8(i8 [[TRUNC]])
; CHECK-NEXT: [[EXT:%.*]] = zext i8 [[REV]] to i32
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[EXT]], [[X]]
; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i8 [[TRUNC]], i8 0
; CHECK-NEXT: ret i8 [[SEL]]
;
%trunc = trunc i32 %x to i8
%rev = call i8 @llvm.bitreverse.i8(i8 %trunc)
%ext = zext i8 %rev to i32
%cmp = icmp eq i32 %ext, %x
%sel = select i1 %cmp, i8 %trunc, i8 0
ret i8 %sel
}

define i16 @select_replacement_loop4(i16 noundef %p_12) {
; CHECK-LABEL: @select_replacement_loop4(
; CHECK-NEXT: [[CMP1:%.*]] = icmp ult i16 [[P_12:%.*]], 2
; CHECK-NEXT: [[AND1:%.*]] = and i16 [[P_12]], 1
; CHECK-NEXT: [[AND2:%.*]] = select i1 [[CMP1]], i16 [[AND1]], i16 0
; CHECK-NEXT: [[CMP2:%.*]] = icmp eq i16 [[AND2]], [[P_12]]
; CHECK-NEXT: [[AND3:%.*]] = select i1 [[CMP2]], i16 [[AND1]], i16 0
; CHECK-NEXT: ret i16 [[AND3]]
;
%cmp1 = icmp ult i16 %p_12, 2
%and1 = and i16 %p_12, 1
%and2 = select i1 %cmp1, i16 %and1, i16 0
%cmp2 = icmp eq i16 %and2, %p_12
%and3 = select i1 %cmp2, i16 %and1, i16 0
ret i16 %and3
}

define ptr @select_replacement_gep_inbounds(ptr %base, i64 %offset) {
; CHECK-LABEL: @select_replacement_gep_inbounds(
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, ptr [[BASE:%.*]], i64 [[OFFSET:%.*]]
Expand Down
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