[llvm] [IndVars] Teach widenLoopCompare to use sext if narrow IV is positive and other operand is already sext. (PR #142703)
Craig Topper via llvm-commits
llvm-commits at lists.llvm.org
Mon Jun 9 21:16:06 PDT 2025
https://github.com/topperc updated https://github.com/llvm/llvm-project/pull/142703
>From da896b7d45c7f727e572e0952d395f6c19179380 Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Tue, 3 Jun 2025 16:48:31 -0700
Subject: [PATCH 1/3] Pre-commit test
---
.../Transforms/IndVarSimplify/iv-cmp-sext.ll | 66 +++++++++++++++++++
1 file changed, 66 insertions(+)
create mode 100644 llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll
diff --git a/llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll b/llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll
new file mode 100644
index 0000000000000..1e42b18268023
--- /dev/null
+++ b/llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll
@@ -0,0 +1,66 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=indvars -S | FileCheck %s
+
+target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
+target triple = "riscv64"
+
+define void @foo(ptr %x, i32 %n) {
+; CHECK-LABEL: define void @foo(
+; CHECK-SAME: ptr [[X:%.*]], i32 [[N:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: [[CMP10:%.*]] = icmp sgt i32 [[N]], 0
+; CHECK-NEXT: br i1 [[CMP10]], label %[[FOR_BODY_PREHEADER:.*]], label %[[FOR_COND_CLEANUP:.*]]
+; CHECK: [[FOR_BODY_PREHEADER]]:
+; CHECK-NEXT: [[WIDE_TRIP_COUNT:%.*]] = zext i32 [[N]] to i64
+; CHECK-NEXT: br label %[[FOR_BODY:.*]]
+; CHECK: [[FOR_COND_CLEANUP_LOOPEXIT:.*]]:
+; CHECK-NEXT: br label %[[FOR_COND_CLEANUP]]
+; CHECK: [[FOR_COND_CLEANUP]]:
+; CHECK-NEXT: ret void
+; CHECK: [[FOR_BODY]]:
+; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], %[[FOR_INC:.*]] ], [ 0, %[[FOR_BODY_PREHEADER]] ]
+; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds nuw i16, ptr [[X]], i64 [[INDVARS_IV]]
+; CHECK-NEXT: [[TMP0:%.*]] = load i16, ptr [[ARRAYIDX]], align 2
+; CHECK-NEXT: [[CONV:%.*]] = sext i16 [[TMP0]] to i32
+; CHECK-NEXT: [[TMP1:%.*]] = zext i32 [[CONV]] to i64
+; CHECK-NEXT: [[CMP1:%.*]] = icmp eq i64 [[INDVARS_IV]], [[TMP1]]
+; CHECK-NEXT: br i1 [[CMP1]], label %[[IF_THEN:.*]], label %[[FOR_INC]]
+; CHECK: [[IF_THEN]]:
+; CHECK-NEXT: store i16 0, ptr [[ARRAYIDX]], align 2
+; CHECK-NEXT: br label %[[FOR_INC]]
+; CHECK: [[FOR_INC]]:
+; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
+; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i64 [[INDVARS_IV_NEXT]], [[WIDE_TRIP_COUNT]]
+; CHECK-NEXT: br i1 [[EXITCOND]], label %[[FOR_BODY]], label %[[FOR_COND_CLEANUP_LOOPEXIT]]
+;
+entry:
+ %cmp10 = icmp sgt i32 %n, 0
+ br i1 %cmp10, label %for.body.preheader, label %for.cond.cleanup
+
+for.body.preheader: ; preds = %entry
+ br label %for.body
+
+for.cond.cleanup.loopexit: ; preds = %for.inc
+ br label %for.cond.cleanup
+
+for.cond.cleanup: ; preds = %for.cond.cleanup.loopexit, %entry
+ ret void
+
+for.body: ; preds = %for.body.preheader, %for.inc
+ %i.011 = phi i32 [ %inc, %for.inc ], [ 0, %for.body.preheader ]
+ %idxprom = zext nneg i32 %i.011 to i64
+ %arrayidx = getelementptr inbounds nuw i16, ptr %x, i64 %idxprom
+ %0 = load i16, ptr %arrayidx, align 2
+ %conv = sext i16 %0 to i32
+ %cmp1 = icmp eq i32 %i.011, %conv
+ br i1 %cmp1, label %if.then, label %for.inc
+
+if.then: ; preds = %for.body
+ store i16 0, ptr %arrayidx, align 2
+ br label %for.inc
+
+for.inc: ; preds = %for.body, %if.then
+ %inc = add nuw nsw i32 %i.011, 1
+ %cmp = icmp slt i32 %inc, %n
+ br i1 %cmp, label %for.body, label %for.cond.cleanup.loopexit
+}
>From d0130a70ad4e11c8229cfb8d5d968cc20ac3abfe Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Tue, 3 Jun 2025 17:03:19 -0700
Subject: [PATCH 2/3] [IndVars] Teach widenLoopCompare to use sext if narrow IV
is positive and other operand is already sext.
This prevents us from ending up with (zext (sext X)). The zext will
require an instruction on targets where zext isn't free like RISC-V.
---
llvm/lib/Transforms/Utils/SimplifyIndVar.cpp | 6 ++++++
llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll | 2 +-
2 files changed, 7 insertions(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index 3fe4621eca70d..6d2266a5a98b5 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -1630,6 +1630,12 @@ bool WidenIV::widenLoopCompare(WidenIV::NarrowIVDefUse DU) {
// Widen the other operand of the compare, if necessary.
if (CastWidth < IVWidth) {
+ // If the narrow IV is always postive and the other operand is sext, widen
+ // using sext so we can combine them. This works for all comparison
+ // predicates.
+ if (DU.NeverNegative && isa<SExtInst>(Op))
+ CmpPreferredSign = true;
+
Value *ExtOp = createExtendInst(Op, WideType, CmpPreferredSign, Cmp);
DU.NarrowUse->replaceUsesOfWith(Op, ExtOp);
}
diff --git a/llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll b/llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll
index 1e42b18268023..34925a6cca955 100644
--- a/llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll
+++ b/llvm/test/Transforms/IndVarSimplify/iv-cmp-sext.ll
@@ -22,7 +22,7 @@ define void @foo(ptr %x, i32 %n) {
; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds nuw i16, ptr [[X]], i64 [[INDVARS_IV]]
; CHECK-NEXT: [[TMP0:%.*]] = load i16, ptr [[ARRAYIDX]], align 2
; CHECK-NEXT: [[CONV:%.*]] = sext i16 [[TMP0]] to i32
-; CHECK-NEXT: [[TMP1:%.*]] = zext i32 [[CONV]] to i64
+; CHECK-NEXT: [[TMP1:%.*]] = sext i32 [[CONV]] to i64
; CHECK-NEXT: [[CMP1:%.*]] = icmp eq i64 [[INDVARS_IV]], [[TMP1]]
; CHECK-NEXT: br i1 [[CMP1]], label %[[IF_THEN:.*]], label %[[FOR_INC]]
; CHECK: [[IF_THEN]]:
>From c853493b254129a5ee2187c5c54b33eb06e2a99c Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Mon, 9 Jun 2025 14:09:46 -0700
Subject: [PATCH 3/3] fixup! Address review comments
---
llvm/lib/Transforms/Utils/SimplifyIndVar.cpp | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index 6d2266a5a98b5..43264cce73719 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -1605,13 +1605,13 @@ bool WidenIV::widenLoopCompare(WidenIV::NarrowIVDefUse DU) {
//
// - The signedness of the IV extension and comparison match
//
- // - The narrow IV is always positive (and thus its sign extension is equal
- // to its zero extension). For instance, let's say we're zero extending
- // %narrow for the following use
+ // - The narrow IV is always non-negative (and thus its sign extension is
+ // equal to its zero extension). For instance, let's say we're zero
+ // extending %narrow for the following use
//
// icmp slt i32 %narrow, %val ... (A)
//
- // and %narrow is always positive. Then
+ // and %narrow is always non-negative. Then
//
// (A) == icmp slt i32 sext(%narrow), sext(%val)
// == icmp slt i32 zext(%narrow), sext(%val)
@@ -1630,10 +1630,10 @@ bool WidenIV::widenLoopCompare(WidenIV::NarrowIVDefUse DU) {
// Widen the other operand of the compare, if necessary.
if (CastWidth < IVWidth) {
- // If the narrow IV is always postive and the other operand is sext, widen
- // using sext so we can combine them. This works for all comparison
- // predicates.
- if (DU.NeverNegative && isa<SExtInst>(Op))
+ // If the narrow IV is always non-negative and the other operand is sext,
+ // widen using sext so we can combine them. This works for all non-signed
+ // comparison predicates.
+ if (DU.NeverNegative && isa<SExtInst>(Op) && !Cmp->isSigned())
CmpPreferredSign = true;
Value *ExtOp = createExtendInst(Op, WideType, CmpPreferredSign, Cmp);
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