[llvm] [InstCombine] Generalize trunc-shift-icmp fold from (1 << Y) to (Pow2 << Y) (PR #169163)
via llvm-commits
llvm-commits at lists.llvm.org
Sat Nov 22 04:45:00 PST 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Pedro Lobo (pedroclobo)
<details>
<summary>Changes</summary>
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
---
Full diff: https://github.com/llvm/llvm-project/pull/169163.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+12-8)
- (modified) llvm/test/Transforms/InstCombine/icmp-trunc.ll (+39-33)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index f153db177cac1..cf6e7315114dc 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -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())) {
diff --git a/llvm/test/Transforms/InstCombine/icmp-trunc.ll b/llvm/test/Transforms/InstCombine/icmp-trunc.ll
index ad76ef7329b0a..6e7b63cfd9589 100644
--- a/llvm/test/Transforms/InstCombine/icmp-trunc.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-trunc.ll
@@ -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 256, %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:%.*]]
``````````
</details>
https://github.com/llvm/llvm-project/pull/169163
More information about the llvm-commits
mailing list