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[InstCombine] Replace one-use select operand based on condition
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InstCombine already performs a fold where X == Y ? f(X) : Z is
transformed to X == Y ? f(Y) : Z if f(Y) simplifies. However,
if f(X) only has one use, then we can always directly replace the
use inside the instruction. To actually be profitable, limit it to
the case where Y is a non-expr constant.

This could be further extended to replace uses further up a one-use
instruction chain, but for now this only looks one level up.

Among other things, this also subsumes D94860.

Differential Revision: https://reviews.llvm.org/D94862
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@nikic
nikic committed Jan 16, 2021
1 parent 32fc323 commit 5238e7b
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Showing 3 changed files with 45 additions and 20 deletions.
17 changes: 16 additions & 1 deletion llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1113,10 +1113,25 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
// replacement cycle.
Value *CmpLHS = Cmp.getOperand(0), *CmpRHS = Cmp.getOperand(1);
if (TrueVal != CmpLHS &&
isGuaranteedNotToBeUndefOrPoison(CmpRHS, SQ.AC, &Sel, &DT))
isGuaranteedNotToBeUndefOrPoison(CmpRHS, SQ.AC, &Sel, &DT)) {
if (Value *V = SimplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, SQ,
/* AllowRefinement */ true))
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
// else. Only do this if CmpRHS is a constant, as profitability is not
// clear for other cases.
// FIXME: The replacement could be performed recursively.
if (isa<Constant>(CmpRHS) && !isa<ConstantExpr>(CmpRHS))
if (auto *I = dyn_cast<Instruction>(TrueVal))
if (I->hasOneUse())
for (Use &U : I->operands())
if (U == CmpLHS) {
replaceUse(U, CmpRHS);
return &Sel;
}
}
if (TrueVal != CmpRHS &&
isGuaranteedNotToBeUndefOrPoison(CmpLHS, SQ.AC, &Sel, &DT))
if (Value *V = SimplifyWithOpReplaced(TrueVal, CmpRHS, CmpLHS, SQ,
Expand Down
44 changes: 27 additions & 17 deletions llvm/test/Transforms/InstCombine/select-binop-cmp.ll
View file
Original file line number Diff line number Diff line change
Expand Up @@ -502,7 +502,7 @@ define i32 @select_xor_icmp_bad_2(i32 %x, i32 %y, i32 %z, i32 %k) {
define i32 @select_xor_icmp_bad_3(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_xor_icmp_bad_3(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 3
; CHECK-NEXT: [[B:%.*]] = xor i32 [[X]], [[Z:%.*]]
; CHECK-NEXT: [[B:%.*]] = xor i32 [[Z:%.*]], 3
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand Down Expand Up @@ -541,7 +541,7 @@ define i32 @select_xor_icmp_bad_5(i32 %x, i32 %y, i32 %z) {
define i32 @select_xor_icmp_bad_6(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_xor_icmp_bad_6(
; CHECK-NEXT: [[A_NOT:%.*]] = icmp eq i32 [[X:%.*]], 1
; CHECK-NEXT: [[B:%.*]] = xor i32 [[X]], [[Z:%.*]]
; CHECK-NEXT: [[B:%.*]] = xor i32 [[Z:%.*]], 1
; CHECK-NEXT: [[C:%.*]] = select i1 [[A_NOT]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -554,7 +554,7 @@ define i32 @select_xor_icmp_bad_6(i32 %x, i32 %y, i32 %z) {
define <2 x i8> @select_xor_icmp_vec_bad(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z) {
; CHECK-LABEL: @select_xor_icmp_vec_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 5, i8 3>
; CHECK-NEXT: [[B:%.*]] = xor <2 x i8> [[X]], [[Z:%.*]]
; CHECK-NEXT: [[B:%.*]] = xor <2 x i8> [[Z:%.*]], <i8 5, i8 3>
; CHECK-NEXT: [[C:%.*]] = select <2 x i1> [[A]], <2 x i8> [[B]], <2 x i8> [[Y:%.*]]
; CHECK-NEXT: ret <2 x i8> [[C]]
;
Expand All @@ -581,7 +581,7 @@ define <2 x i8> @select_xor_icmp_vec_undef(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z
define i32 @select_mul_icmp_bad(i32 %x, i32 %y, i32 %z, i32 %k) {
; CHECK-LABEL: @select_mul_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 3
; CHECK-NEXT: [[B:%.*]] = mul i32 [[X]], [[Z:%.*]]
; CHECK-NEXT: [[B:%.*]] = mul i32 [[Z:%.*]], 3
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -594,7 +594,7 @@ define i32 @select_mul_icmp_bad(i32 %x, i32 %y, i32 %z, i32 %k) {
define i32 @select_add_icmp_bad(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_add_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 1
; CHECK-NEXT: [[B:%.*]] = add i32 [[X]], [[Z:%.*]]
; CHECK-NEXT: [[B:%.*]] = add i32 [[Z:%.*]], 1
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -619,7 +619,7 @@ define i32 @select_and_icmp_zero(i32 %x, i32 %y, i32 %z) {
define i32 @select_or_icmp_bad(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_or_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 3
; CHECK-NEXT: [[B:%.*]] = or i32 [[X]], [[Z:%.*]]
; CHECK-NEXT: [[B:%.*]] = or i32 [[Z:%.*]], 3
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand Down Expand Up @@ -940,7 +940,7 @@ define float @select_fsub_fcmp_bad_2(float %x, float %y, float %z) {
define i32 @select_sub_icmp_bad(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_sub_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 0
; CHECK-NEXT: [[B:%.*]] = sub i32 [[X]], [[Z:%.*]]
; CHECK-NEXT: [[B:%.*]] = sub i32 0, [[Z:%.*]]
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -953,7 +953,7 @@ define i32 @select_sub_icmp_bad(i32 %x, i32 %y, i32 %z) {
define i32 @select_sub_icmp_bad_2(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_sub_icmp_bad_2(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 1
; CHECK-NEXT: [[B:%.*]] = sub i32 [[Z:%.*]], [[X]]
; CHECK-NEXT: [[B:%.*]] = add i32 [[Z:%.*]], -1
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand Down Expand Up @@ -1022,7 +1022,7 @@ define i32 @select_sub_icmp_bad_5(i32 %x, i32 %y, i32 %z, i32 %k) {
define i32 @select_shl_icmp_bad(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_shl_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 1
; CHECK-NEXT: [[B:%.*]] = shl i32 [[Z:%.*]], [[X]]
; CHECK-NEXT: [[B:%.*]] = shl i32 [[Z:%.*]], 1
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -1035,7 +1035,7 @@ define i32 @select_shl_icmp_bad(i32 %x, i32 %y, i32 %z) {
define i32 @select_lshr_icmp_bad(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_lshr_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 1
; CHECK-NEXT: [[B:%.*]] = lshr i32 [[Z:%.*]], [[X]]
; CHECK-NEXT: [[B:%.*]] = lshr i32 [[Z:%.*]], 1
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -1048,7 +1048,7 @@ define i32 @select_lshr_icmp_bad(i32 %x, i32 %y, i32 %z) {
define i32 @select_ashr_icmp_bad(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_ashr_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 1
; CHECK-NEXT: [[B:%.*]] = ashr i32 [[Z:%.*]], [[X]]
; CHECK-NEXT: [[B:%.*]] = ashr i32 [[Z:%.*]], 1
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -1061,7 +1061,7 @@ define i32 @select_ashr_icmp_bad(i32 %x, i32 %y, i32 %z) {
define i32 @select_udiv_icmp_bad(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_udiv_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 3
; CHECK-NEXT: [[B:%.*]] = udiv i32 [[Z:%.*]], [[X]]
; CHECK-NEXT: [[B:%.*]] = udiv i32 [[Z:%.*]], 3
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -1074,7 +1074,7 @@ define i32 @select_udiv_icmp_bad(i32 %x, i32 %y, i32 %z) {
define i32 @select_sdiv_icmp_bad(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_sdiv_icmp_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq i32 [[X:%.*]], 3
; CHECK-NEXT: [[B:%.*]] = sdiv i32 [[Z:%.*]], [[X]]
; CHECK-NEXT: [[B:%.*]] = sdiv i32 [[Z:%.*]], 3
; CHECK-NEXT: [[C:%.*]] = select i1 [[A]], i32 [[B]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[C]]
;
Expand All @@ -1084,10 +1084,11 @@ define i32 @select_sdiv_icmp_bad(i32 %x, i32 %y, i32 %z) {
ret i32 %C
}

; Can replace %x with 0, because sub is only used in the select.
define i32 @select_replace_one_use(i32 %x, i32 %y) {
; CHECK-LABEL: @select_replace_one_use(
; CHECK-NEXT: [[C:%.*]] = icmp eq i32 [[X:%.*]], 0
; CHECK-NEXT: [[SUB:%.*]] = sub i32 [[X]], [[Y:%.*]]
; CHECK-NEXT: [[SUB:%.*]] = sub i32 0, [[Y:%.*]]
; CHECK-NEXT: [[S:%.*]] = select i1 [[C]], i32 [[SUB]], i32 [[Y]]
; CHECK-NEXT: ret i32 [[S]]
;
Expand All @@ -1097,6 +1098,7 @@ define i32 @select_replace_one_use(i32 %x, i32 %y) {
ret i32 %s
}

; Can not replace %x with 0, because %sub has other uses as well.
define i32 @select_replace_multi_use(i32 %x, i32 %y) {
; CHECK-LABEL: @select_replace_multi_use(
; CHECK-NEXT: [[C:%.*]] = icmp eq i32 [[X:%.*]], 0
Expand All @@ -1112,11 +1114,11 @@ define i32 @select_replace_multi_use(i32 %x, i32 %y) {
ret i32 %s
}

; Case where the replacement allows the instruction to fold away.
define i32 @select_replace_fold(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_replace_fold(
; CHECK-NEXT: [[C:%.*]] = icmp eq i32 [[X:%.*]], 0
; CHECK-NEXT: [[FSHR:%.*]] = call i32 @llvm.fshr.i32(i32 [[Y:%.*]], i32 [[Z:%.*]], i32 [[X]])
; CHECK-NEXT: [[S:%.*]] = select i1 [[C]], i32 [[FSHR]], i32 [[Y]]
; CHECK-NEXT: [[S:%.*]] = select i1 [[C]], i32 [[Z:%.*]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[S]]
;
%c = icmp eq i32 %x, 0
Expand All @@ -1125,10 +1127,11 @@ define i32 @select_replace_fold(i32 %x, i32 %y, i32 %z) {
ret i32 %s
}

; Case where %x can be replaced by 0 in multiple operands of the same instr.
define i32 @select_replace_multiple_ops(i32 %x, i32 %y) {
; CHECK-LABEL: @select_replace_multiple_ops(
; CHECK-NEXT: [[C:%.*]] = icmp eq i32 [[X:%.*]], 0
; CHECK-NEXT: [[CALL:%.*]] = call i32 @dummy_call(i32 [[X]], i32 [[X]], i32 [[X]])
; CHECK-NEXT: [[CALL:%.*]] = call i32 @dummy_call(i32 0, i32 0, i32 0)
; CHECK-NEXT: [[S:%.*]] = select i1 [[C]], i32 [[CALL]], i32 [[Y:%.*]]
; CHECK-NEXT: ret i32 [[S]]
;
Expand All @@ -1138,6 +1141,8 @@ define i32 @select_replace_multiple_ops(i32 %x, i32 %y) {
ret i32 %s
}

; Case where the use of %x is in a nested instruction.
; FIXME: We only perform replacements one level up right now.
define i32 @select_replace_nested(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_replace_nested(
; CHECK-NEXT: [[C:%.*]] = icmp eq i32 [[X:%.*]], 0
Expand All @@ -1153,6 +1158,9 @@ define i32 @select_replace_nested(i32 %x, i32 %y, i32 %z) {
ret i32 %s
}

; Do not replace with constant expressions. The profitability in this case is
; unclear, and such replacements have historically lead to infinite combine
; loops.
define i32 @select_replace_constexpr(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @select_replace_constexpr(
; CHECK-NEXT: [[C:%.*]] = icmp eq i32 [[X:%.*]], ptrtoint (i32* @g to i32)
Expand All @@ -1166,6 +1174,8 @@ define i32 @select_replace_constexpr(i32 %x, i32 %y, i32 %z) {
ret i32 %s
}

; Don't replace with a potentially undef constant, as undef could evaluate
; to different values for both uses.
define <2 x i32> @select_replace_undef(<2 x i32> %x, <2 x i32> %y) {
; CHECK-LABEL: @select_replace_undef(
; CHECK-NEXT: [[C:%.*]] = icmp eq <2 x i32> [[X:%.*]], <i32 0, i32 undef>
Expand Down
4 changes: 2 additions & 2 deletions llvm/test/Transforms/InstCombine/select-safe-transforms.ll
View file
Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,7 @@ define i1 @cond_eq_and(i8 %X, i8 %Y, i8 noundef %C) {
define i1 @cond_eq_and_const(i8 %X, i8 %Y) {
; CHECK-LABEL: @cond_eq_and_const(
; CHECK-NEXT: [[COND:%.*]] = icmp eq i8 [[X:%.*]], 10
; CHECK-NEXT: [[LHS:%.*]] = icmp ult i8 [[X]], [[Y:%.*]]
; CHECK-NEXT: [[LHS:%.*]] = icmp ugt i8 [[Y:%.*]], 10
; CHECK-NEXT: [[RES:%.*]] = select i1 [[COND]], i1 [[LHS]], i1 false
; CHECK-NEXT: ret i1 [[RES]]
;
Expand All @@ -43,7 +43,7 @@ define i1 @cond_eq_or(i8 %X, i8 %Y, i8 noundef %C) {
define i1 @cond_eq_or_const(i8 %X, i8 %Y) {
; CHECK-LABEL: @cond_eq_or_const(
; CHECK-NEXT: [[COND:%.*]] = icmp ne i8 [[X:%.*]], 10
; CHECK-NEXT: [[LHS:%.*]] = icmp ult i8 [[X]], [[Y:%.*]]
; CHECK-NEXT: [[LHS:%.*]] = icmp ugt i8 [[Y:%.*]], 10
; CHECK-NEXT: [[RES:%.*]] = select i1 [[COND]], i1 true, i1 [[LHS]]
; CHECK-NEXT: ret i1 [[RES]]
;
Expand Down

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