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[InstCombine] Generalize trunc-shift-icmp fold from (1 << Y) to (Pow2 << Y) (#169163)

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Extends the `icmp(trunc(shl))` fold to handle any power of 2 constant as
the shift base, not just 1. This generalizes the following patterns by
adjusting the comparison offsets by `log2(Pow2)`.

```llvm
(trunc (1 << Y) to iN) == 0    --> Y u>= N
(trunc (1 << Y) to iN) != 0    --> Y u<  N
(trunc (1 << Y) to iN) == 2**C --> Y ==  C
(trunc (1 << Y) to iN) != 2**C --> Y !=  C

; to

(trunc (Pow2 << Y) to iN) == 0    --> Y u>= N - log2(Pow2)
(trunc (Pow2 << Y) to iN) != 0    --> Y u<  N - log2(Pow2)
(trunc (Pow2 << Y) to iN) == 2**C --> Y ==  C - log2(Pow2)
(trunc (Pow2 << Y) to iN) != 2**C --> Y !=  C - log2(Pow2)
```

Proof: https://alive2.llvm.org/ce/z/2zwTkp
This commit is contained in:
Pedro Lobo 2025-11-22 15:44:06 +00:00 committed by GitHub
parent cc4dd015ad
commit e8af134bb7
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GPG Key ID: B5690EEEBB952194
2 changed files with 51 additions and 41 deletions
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20
llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
View File

@ -1465,20 +1465,24 @@ Instruction *InstCombinerImpl::foldICmpTruncConstant(ICmpInst &Cmp,
ConstantInt::get(V->getType(), 1));
}
// TODO: Handle any shifted constant by subtracting trailing zeros.
// TODO: Handle non-equality predicates.
Value *Y;
if (Cmp.isEquality() && match(X, m_Shl(m_One(), m_Value(Y)))) {
// (trunc (1 << Y) to iN) == 0 --> Y u>= N
// (trunc (1 << Y) to iN) != 0 --> Y u< N
const APInt *Pow2;
if (Cmp.isEquality() && match(X, m_Shl(m_Power2(Pow2), m_Value(Y))) &&
DstBits > Pow2->logBase2()) {
// (trunc (Pow2 << Y) to iN) == 0 --> Y u>= N - log2(Pow2)
// (trunc (Pow2 << Y) to iN) != 0 --> Y u< N - log2(Pow2)
// iff N > log2(Pow2)
if (C.isZero()) {
auto NewPred = (Pred == Cmp.ICMP_EQ) ? Cmp.ICMP_UGE : Cmp.ICMP_ULT;
return new ICmpInst(NewPred, Y, ConstantInt::get(SrcTy, DstBits));
return new ICmpInst(NewPred, Y,
ConstantInt::get(SrcTy, DstBits - Pow2->logBase2()));
}
// (trunc (1 << Y) to iN) == 2**C --> Y == C
// (trunc (1 << Y) to iN) != 2**C --> Y != C
// (trunc (Pow2 << Y) to iN) == 2**C --> Y == C - log2(Pow2)
// (trunc (Pow2 << Y) to iN) != 2**C --> Y != C - log2(Pow2)
if (C.isPowerOf2())
return new ICmpInst(Pred, Y, ConstantInt::get(SrcTy, C.logBase2()));
return new ICmpInst(
Pred, Y, ConstantInt::get(SrcTy, C.logBase2() - Pow2->logBase2()));
}
if (Cmp.isEquality() && (Trunc->hasOneUse() || Trunc->hasNoUnsignedWrap())) {

72
llvm/test/Transforms/InstCombine/icmp-trunc.ll
View File

@ -407,13 +407,9 @@ define i1 @shl1_trunc_ne0_use2(i37 %a) {
ret i1 %r
}
; TODO: A > 4
define i1 @shl2_trunc_eq0(i9 %a) {
; CHECK-LABEL: @shl2_trunc_eq0(
; CHECK-NEXT: [[SHL:%.*]] = shl i9 2, [[A:%.*]]
; CHECK-NEXT: [[T:%.*]] = trunc i9 [[SHL]] to i6
; CHECK-NEXT: [[R:%.*]] = icmp eq i6 [[T]], 0
; CHECK-NEXT: [[R:%.*]] = icmp ugt i9 [[A:%.*]], 4
; CHECK-NEXT: ret i1 [[R]]
;
%shl = shl i9 2, %a
@ -424,9 +420,7 @@ define i1 @shl2_trunc_eq0(i9 %a) {
define i1 @shl2_trunc_ne0(i9 %a) {
; CHECK-LABEL: @shl2_trunc_ne0(
; CHECK-NEXT: [[SHL:%.*]] = shl i9 2, [[A:%.*]]
; CHECK-NEXT: [[T:%.*]] = trunc i9 [[SHL]] to i6
; CHECK-NEXT: [[R:%.*]] = icmp ne i6 [[T]], 0
; CHECK-NEXT: [[R:%.*]] = icmp ult i9 [[A:%.*]], 5
; CHECK-NEXT: ret i1 [[R]]
;
%shl = shl i9 2, %a
@ -450,9 +444,7 @@ define i1 @shl3_trunc_eq0(i9 %a) {
define <2 x i1> @shl4_trunc_ne0(<2 x i8> %a) {
; CHECK-LABEL: @shl4_trunc_ne0(
; CHECK-NEXT: [[SHL:%.*]] = shl <2 x i8> <i8 4, i8 poison>, [[A:%.*]]
; CHECK-NEXT: [[T:%.*]] = trunc <2 x i8> [[SHL]] to <2 x i5>
; CHECK-NEXT: [[R:%.*]] = icmp ne <2 x i5> [[T]], zeroinitializer
; CHECK-NEXT: [[R:%.*]] = icmp ult <2 x i8> [[A:%.*]], splat (i8 3)
; CHECK-NEXT: ret <2 x i1> [[R]]
;
%shl = shl <2 x i8> <i8 4, i8 poison>, %a
@ -461,6 +453,16 @@ define <2 x i1> @shl4_trunc_ne0(<2 x i8> %a) {
ret <2 x i1> %r
}
define i1 @shl5_trunc_ne0(i9 %a) {
; CHECK-LABEL: @shl5_trunc_ne0(
; CHECK-NEXT: [[R:%.*]] = icmp ult i9 [[A:%.*]], 4
; CHECK-NEXT: ret i1 [[R]]
;
%shl = shl i9 4, %a
%t = trunc i9 %shl to i6
%r = icmp ne i6 %t, 0
ret i1 %r
}
; TODO: A < 5
@ -507,17 +509,9 @@ define i1 @shl1_trunc_ne32(i8 %a) {
}
define i1 @shl2_trunc_eq8_i32(i32 %a) {
; DL64-LABEL: @shl2_trunc_eq8_i32(
; DL64-NEXT: [[SHL:%.*]] = shl i32 2, [[A:%.*]]
; DL64-NEXT: [[TMP1:%.*]] = and i32 [[SHL]], 65534
; DL64-NEXT: [[R:%.*]] = icmp eq i32 [[TMP1]], 8
; DL64-NEXT: ret i1 [[R]]
;
; DL8-LABEL: @shl2_trunc_eq8_i32(
; DL8-NEXT: [[SHL:%.*]] = shl i32 2, [[A:%.*]]
; DL8-NEXT: [[T:%.*]] = trunc i32 [[SHL]] to i16
; DL8-NEXT: [[R:%.*]] = icmp eq i16 [[T]], 8
; DL8-NEXT: ret i1 [[R]]
; CHECK-LABEL: @shl2_trunc_eq8_i32(
; CHECK-NEXT: [[R:%.*]] = icmp eq i32 [[A:%.*]], 2
; CHECK-NEXT: ret i1 [[R]]
;
%shl = shl i32 2, %a
%t = trunc i32 %shl to i16
@ -526,17 +520,9 @@ define i1 @shl2_trunc_eq8_i32(i32 %a) {
}
define i1 @shl2_trunc_ne8_i32(i32 %a) {
; DL64-LABEL: @shl2_trunc_ne8_i32(
; DL64-NEXT: [[SHL:%.*]] = shl i32 2, [[A:%.*]]
; DL64-NEXT: [[TMP1:%.*]] = and i32 [[SHL]], 65534
; DL64-NEXT: [[R:%.*]] = icmp ne i32 [[TMP1]], 8
; DL64-NEXT: ret i1 [[R]]
;
; DL8-LABEL: @shl2_trunc_ne8_i32(
; DL8-NEXT: [[SHL:%.*]] = shl i32 2, [[A:%.*]]
; DL8-NEXT: [[T:%.*]] = trunc i32 [[SHL]] to i16
; DL8-NEXT: [[R:%.*]] = icmp ne i16 [[T]], 8
; DL8-NEXT: ret i1 [[R]]
; CHECK-LABEL: @shl2_trunc_ne8_i32(
; CHECK-NEXT: [[R:%.*]] = icmp ne i32 [[A:%.*]], 2
; CHECK-NEXT: ret i1 [[R]]
;
%shl = shl i32 2, %a
%t = trunc i32 %shl to i16
@ -544,6 +530,26 @@ define i1 @shl2_trunc_ne8_i32(i32 %a) {
ret i1 %r
}
define i1 @neg_shl2_trunc_eq0_i8(i8 %a) {
; CHECK-LABEL: @neg_shl2_trunc_eq0_i8(
; CHECK-NEXT: ret i1 true
;
%shl = shl i8 128, %a
%t = trunc i8 %shl to i6
%r = icmp eq i6 %t, 0
ret i1 %r
}
define i1 @neg_shl2_trunc_ne0_i8(i8 %a) {
; CHECK-LABEL: @neg_shl2_trunc_ne0_i8(
; CHECK-NEXT: ret i1 false
;
%shl = shl i8 128, %a
%t = trunc i8 %shl to i6
%r = icmp ne i6 %t, 0
ret i1 %r
}
define i1 @shl1_trunc_sgt4(i32 %a) {
; CHECK-LABEL: @shl1_trunc_sgt4(
; CHECK-NEXT: [[SHL:%.*]] = shl nuw i32 1, [[A:%.*]]