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[InstCombine] Fold icmp of truncated left shift

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(trunc (1 << Y) to iN) == 0 --> Y u>= N
(trunc (1 << Y) to iN) != 0 --> Y u<  N

These can be generalized in several ways as noted by the TODO
items, but this handles the pattern in the motivating bug report.

Fixes #51889

Differential Revision: https://reviews.llvm.org/D115480
This commit is contained in:
Sanjay Patel
2022-09-08 10:44:49 -04:00
parent 39fcb4a268
commit d9e1f9d759
2 changed files with 25 additions and 13 deletions
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12
llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
View File

@@ -1551,6 +1551,18 @@ Instruction *InstCombinerImpl::foldICmpTruncConstant(ICmpInst &Cmp,
Type *SrcTy = X->getType();
unsigned DstBits = Trunc->getType()->getScalarSizeInBits(),
SrcBits = SrcTy->getScalarSizeInBits();
// (trunc (1 << Y) to iN) == 0 --> Y u>= N
// (trunc (1 << Y) to iN) != 0 --> Y u< N
// TODO: Handle any shifted constant by subtracting trailing zeros.
// TODO: Handle non-equality predicates.
// TODO: Handle compare to power-of-2 (non-zero) constant.
Value *Y;
if (Cmp.isEquality() && C.isZero() && match(X, m_Shl(m_One(), m_Value(Y)))) {
auto NewPred = Pred == Cmp.ICMP_EQ ? Cmp.ICMP_UGE : Cmp.ICMP_ULT;
return new ICmpInst(NewPred, Y, ConstantInt::get(SrcTy, DstBits));
}
if (Cmp.isEquality() && Trunc->hasOneUse()) {
// Canonicalize to a mask and wider compare if the wide type is suitable:
// (trunc X to i8) == C --> (X & 0xff) == (zext C)

26
llvm/test/Transforms/InstCombine/icmp-trunc.ll
View File

@@ -356,15 +356,9 @@ define i1 @trunc_ne_i64_i10(i64 %x) {
}
define i1 @shl1_trunc_eq0(i32 %a) {
; DL64-LABEL: @shl1_trunc_eq0(
; DL64-NEXT: [[R:%.*]] = icmp ugt i32 [[A:%.*]], 15
; DL64-NEXT: ret i1 [[R]]
;
; DL8-LABEL: @shl1_trunc_eq0(
; DL8-NEXT: [[SHL:%.*]] = shl i32 1, [[A:%.*]]
; DL8-NEXT: [[T:%.*]] = trunc i32 [[SHL]] to i16
; DL8-NEXT: [[R:%.*]] = icmp eq i16 [[T]], 0
; DL8-NEXT: ret i1 [[R]]
; CHECK-LABEL: @shl1_trunc_eq0(
; CHECK-NEXT: [[R:%.*]] = icmp ugt i32 [[A:%.*]], 15
; CHECK-NEXT: ret i1 [[R]]
;
%shl = shl i32 1, %a
%t = trunc i32 %shl to i16
@@ -374,9 +368,7 @@ define i1 @shl1_trunc_eq0(i32 %a) {
define <2 x i1> @shl1_trunc_ne0(<2 x i8> %a) {
; CHECK-LABEL: @shl1_trunc_ne0(
; CHECK-NEXT: [[SHL:%.*]] = shl <2 x i8> <i8 1, 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:%.*]], <i8 5, i8 5>
; CHECK-NEXT: ret <2 x i1> [[R]]
;
%shl = shl <2 x i8> <i8 1, i8 poison>, %a
@@ -404,7 +396,7 @@ define i1 @shl1_trunc_ne0_use2(i37 %a) {
; CHECK-NEXT: [[SHL:%.*]] = shl i37 1, [[A:%.*]]
; CHECK-NEXT: [[T:%.*]] = trunc i37 [[SHL]] to i8
; CHECK-NEXT: call void @use(i8 [[T]])
; CHECK-NEXT: [[R:%.*]] = icmp ne i8 [[T]], 0
; CHECK-NEXT: [[R:%.*]] = icmp ult i37 [[A]], 8
; CHECK-NEXT: ret i1 [[R]]
;
%shl = shl i37 1, %a
@@ -414,6 +406,8 @@ 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:%.*]]
@@ -427,6 +421,8 @@ define i1 @shl2_trunc_eq0(i9 %a) {
ret i1 %r
}
; TODO: A < 5
define i1 @shl1_trunc_sgt0(i9 %a) {
; CHECK-LABEL: @shl1_trunc_sgt0(
; CHECK-NEXT: [[SHL:%.*]] = shl i9 1, [[A:%.*]]
@@ -440,6 +436,8 @@ define i1 @shl1_trunc_sgt0(i9 %a) {
ret i1 %r
}
; TODO: A == 0
define i1 @shl1_trunc_eq1(i64 %a) {
; CHECK-LABEL: @shl1_trunc_eq1(
; CHECK-NEXT: [[SHL:%.*]] = shl i64 1, [[A:%.*]]
@@ -455,6 +453,8 @@ define i1 @shl1_trunc_eq1(i64 %a) {
ret i1 %r
}
; TODO: A != 5
define i1 @shl1_trunc_ne32(i8 %a) {
; CHECK-LABEL: @shl1_trunc_ne32(
; CHECK-NEXT: [[SHL:%.*]] = shl i8 1, [[A:%.*]]