[llvm] [InstCombine] Fix multi-use handling for multi-GEP rewrite (PR #146689)
Nikita Popov via llvm-commits
llvm-commits at lists.llvm.org
Wed Jul 2 06:15:03 PDT 2025
https://github.com/nikic created https://github.com/llvm/llvm-project/pull/146689
If we're expanding offsets for a chain of GEPs in RewriteGEPs mode, we should also rewrite GEPs that have one-use themselves, but are kept alive by a multi-use GEP later in the chain.
For the sake of simplicity, I've changed this to just skip the one-use condition entirely (which will perform an unnecessary rewrite of a no longer used GEP, but shouldn't otherwise matter).
>From 564840820f2174b8b6a3f1c15463c9262b470503 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Wed, 2 Jul 2025 15:01:50 +0200
Subject: [PATCH 1/2] Add extra multi-use test
---
llvm/test/Transforms/InstCombine/sub-gep.ll | 60 +++++++++++++++++++++
1 file changed, 60 insertions(+)
diff --git a/llvm/test/Transforms/InstCombine/sub-gep.ll b/llvm/test/Transforms/InstCombine/sub-gep.ll
index 45a30350aafe1..5361b95dd4e0a 100644
--- a/llvm/test/Transforms/InstCombine/sub-gep.ll
+++ b/llvm/test/Transforms/InstCombine/sub-gep.ll
@@ -943,6 +943,66 @@ define i64 @multiple_geps_two_chains_gep_base(ptr %base, i64 %base.idx, i64 %idx
ret i64 %d
}
+define i64 @multiple_geps_two_chains_multi_use(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4) {
+; CHECK-LABEL: @multiple_geps_two_chains_multi_use(
+; CHECK-NEXT: [[P1:%.*]] = getelementptr inbounds i32, ptr [[BASE:%.*]], i64 [[IDX1:%.*]]
+; CHECK-NEXT: [[P2_IDX:%.*]] = shl nsw i64 [[IDX2:%.*]], 2
+; CHECK-NEXT: [[P2:%.*]] = getelementptr inbounds i8, ptr [[P1]], i64 [[P2_IDX]]
+; CHECK-NEXT: [[P3:%.*]] = getelementptr inbounds i32, ptr [[BASE]], i64 [[IDX3:%.*]]
+; CHECK-NEXT: [[P4_IDX:%.*]] = shl nsw i64 [[IDX4:%.*]], 2
+; CHECK-NEXT: [[P4:%.*]] = getelementptr inbounds i8, ptr [[P3]], i64 [[P4_IDX]]
+; CHECK-NEXT: call void @use(ptr [[P2]])
+; CHECK-NEXT: call void @use(ptr [[P4]])
+; CHECK-NEXT: [[P1_IDX1:%.*]] = add i64 [[IDX1]], [[IDX2]]
+; CHECK-NEXT: [[P3_IDX2:%.*]] = add i64 [[IDX3]], [[IDX4]]
+; CHECK-NEXT: [[TMP1:%.*]] = sub i64 [[P1_IDX1]], [[P3_IDX2]]
+; CHECK-NEXT: [[GEPDIFF:%.*]] = shl i64 [[TMP1]], 2
+; CHECK-NEXT: ret i64 [[GEPDIFF]]
+;
+ %p1 = getelementptr inbounds i32, ptr %base, i64 %idx1
+ %p2 = getelementptr inbounds i32, ptr %p1, i64 %idx2
+ %p3 = getelementptr inbounds i32, ptr %base, i64 %idx3
+ %p4 = getelementptr inbounds i32, ptr %p3, i64 %idx4
+ call void @use(ptr %p2)
+ call void @use(ptr %p4)
+ %i1 = ptrtoint ptr %p4 to i64
+ %i2 = ptrtoint ptr %p2 to i64
+ %d = sub i64 %i2, %i1
+ ret i64 %d
+}
+
+define i64 @multiple_geps_two_chains_partial_multi_use(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4, i64 %idx5, i64 %idx6) {
+; CHECK-LABEL: @multiple_geps_two_chains_partial_multi_use(
+; CHECK-NEXT: [[P1:%.*]] = getelementptr inbounds i32, ptr [[BASE:%.*]], i64 [[IDX1:%.*]]
+; CHECK-NEXT: [[P2_IDX:%.*]] = shl nsw i64 [[IDX2:%.*]], 2
+; CHECK-NEXT: [[P2:%.*]] = getelementptr inbounds i8, ptr [[P1]], i64 [[P2_IDX]]
+; CHECK-NEXT: [[P5:%.*]] = getelementptr inbounds i32, ptr [[BASE]], i64 [[IDX5:%.*]]
+; CHECK-NEXT: [[P4_IDX:%.*]] = shl nsw i64 [[IDX4:%.*]], 2
+; CHECK-NEXT: [[P4:%.*]] = getelementptr inbounds i8, ptr [[P5]], i64 [[P4_IDX]]
+; CHECK-NEXT: call void @use(ptr [[P2]])
+; CHECK-NEXT: call void @use(ptr [[P4]])
+; CHECK-NEXT: [[P1_IDX1:%.*]] = add i64 [[IDX1]], [[IDX2]]
+; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[P1_IDX1]], [[IDX3:%.*]]
+; CHECK-NEXT: [[P4_IDX2:%.*]] = add i64 [[IDX5]], [[IDX4]]
+; CHECK-NEXT: [[TMP2:%.*]] = add i64 [[P4_IDX2]], [[IDX6:%.*]]
+; CHECK-NEXT: [[TMP3:%.*]] = sub i64 [[TMP1]], [[TMP2]]
+; CHECK-NEXT: [[GEPDIFF:%.*]] = shl i64 [[TMP3]], 2
+; CHECK-NEXT: ret i64 [[GEPDIFF]]
+;
+ %p1 = getelementptr inbounds i32, ptr %base, i64 %idx1
+ %p2 = getelementptr inbounds i32, ptr %p1, i64 %idx2
+ %p3 = getelementptr inbounds i32, ptr %p2, i64 %idx3
+ %p4 = getelementptr inbounds i32, ptr %base, i64 %idx4
+ %p5 = getelementptr inbounds i32, ptr %p4, i64 %idx5
+ %p6 = getelementptr inbounds i32, ptr %p5, i64 %idx6
+ call void @use(ptr %p2)
+ call void @use(ptr %p5)
+ %i1 = ptrtoint ptr %p6 to i64
+ %i2 = ptrtoint ptr %p3 to i64
+ %d = sub i64 %i2, %i1
+ ret i64 %d
+}
+
define i64 @multiple_geps_inbounds(ptr %base, i64 %idx, i64 %idx2) {
; CHECK-LABEL: @multiple_geps_inbounds(
; CHECK-NEXT: [[D:%.*]] = add nsw i64 [[IDX:%.*]], [[IDX2:%.*]]
>From dbf821be487048fe5bc4411ea3a4956777713b95 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Wed, 2 Jul 2025 15:11:29 +0200
Subject: [PATCH 2/2] fix multi-use handling when expanding multiple gep
offsets
---
.../InstCombine/InstructionCombining.cpp | 5 +--
llvm/test/Transforms/InstCombine/sub-gep.ll | 44 ++++++++++---------
2 files changed, 26 insertions(+), 23 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 806b38874b450..91a1b61ddc483 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -205,9 +205,8 @@ Value *InstCombinerImpl::EmitGEPOffset(GEPOperator *GEP, bool RewriteGEP) {
Builder.SetInsertPoint(Inst);
Value *Offset = EmitGEPOffset(GEP);
- // If a non-trivial GEP has other uses, rewrite it to avoid duplicating
- // the offset arithmetic.
- if (Inst && !GEP->hasOneUse() && !GEP->hasAllConstantIndices() &&
+ // Rewrite non-trivial GEPs to avoid duplicating the offset arithmetic.
+ if (Inst && !GEP->hasAllConstantIndices() &&
!GEP->getSourceElementType()->isIntegerTy(8)) {
replaceInstUsesWith(
*Inst, Builder.CreateGEP(Builder.getInt8Ty(), GEP->getPointerOperand(),
diff --git a/llvm/test/Transforms/InstCombine/sub-gep.ll b/llvm/test/Transforms/InstCombine/sub-gep.ll
index 5361b95dd4e0a..993a06ad1780f 100644
--- a/llvm/test/Transforms/InstCombine/sub-gep.ll
+++ b/llvm/test/Transforms/InstCombine/sub-gep.ll
@@ -945,18 +945,19 @@ define i64 @multiple_geps_two_chains_gep_base(ptr %base, i64 %base.idx, i64 %idx
define i64 @multiple_geps_two_chains_multi_use(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4) {
; CHECK-LABEL: @multiple_geps_two_chains_multi_use(
-; CHECK-NEXT: [[P1:%.*]] = getelementptr inbounds i32, ptr [[BASE:%.*]], i64 [[IDX1:%.*]]
; CHECK-NEXT: [[P2_IDX:%.*]] = shl nsw i64 [[IDX2:%.*]], 2
-; CHECK-NEXT: [[P2:%.*]] = getelementptr inbounds i8, ptr [[P1]], i64 [[P2_IDX]]
-; CHECK-NEXT: [[P3:%.*]] = getelementptr inbounds i32, ptr [[BASE]], i64 [[IDX3:%.*]]
+; CHECK-NEXT: [[P2:%.*]] = getelementptr inbounds i8, ptr [[P1:%.*]], i64 [[P2_IDX]]
; CHECK-NEXT: [[P4_IDX:%.*]] = shl nsw i64 [[IDX4:%.*]], 2
-; CHECK-NEXT: [[P4:%.*]] = getelementptr inbounds i8, ptr [[P3]], i64 [[P4_IDX]]
-; CHECK-NEXT: call void @use(ptr [[P2]])
+; CHECK-NEXT: [[P5:%.*]] = getelementptr inbounds i8, ptr [[P2]], i64 [[P4_IDX]]
+; CHECK-NEXT: [[P3_IDX:%.*]] = shl nsw i64 [[IDX3:%.*]], 2
+; CHECK-NEXT: [[P3:%.*]] = getelementptr inbounds i8, ptr [[P1]], i64 [[P3_IDX]]
+; CHECK-NEXT: [[P4_IDX1:%.*]] = shl nsw i64 [[IDX5:%.*]], 2
+; CHECK-NEXT: [[P4:%.*]] = getelementptr inbounds i8, ptr [[P3]], i64 [[P4_IDX1]]
+; CHECK-NEXT: call void @use(ptr [[P5]])
; CHECK-NEXT: call void @use(ptr [[P4]])
-; CHECK-NEXT: [[P1_IDX1:%.*]] = add i64 [[IDX1]], [[IDX2]]
-; CHECK-NEXT: [[P3_IDX2:%.*]] = add i64 [[IDX3]], [[IDX4]]
-; CHECK-NEXT: [[TMP1:%.*]] = sub i64 [[P1_IDX1]], [[P3_IDX2]]
-; CHECK-NEXT: [[GEPDIFF:%.*]] = shl i64 [[TMP1]], 2
+; CHECK-NEXT: [[TMP1:%.*]] = add nsw i64 [[P2_IDX]], [[P4_IDX]]
+; CHECK-NEXT: [[TMP2:%.*]] = add nsw i64 [[P3_IDX]], [[P4_IDX1]]
+; CHECK-NEXT: [[GEPDIFF:%.*]] = sub nsw i64 [[TMP1]], [[TMP2]]
; CHECK-NEXT: ret i64 [[GEPDIFF]]
;
%p1 = getelementptr inbounds i32, ptr %base, i64 %idx1
@@ -973,20 +974,23 @@ define i64 @multiple_geps_two_chains_multi_use(ptr %base, i64 %idx1, i64 %idx2,
define i64 @multiple_geps_two_chains_partial_multi_use(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4, i64 %idx5, i64 %idx6) {
; CHECK-LABEL: @multiple_geps_two_chains_partial_multi_use(
-; CHECK-NEXT: [[P1:%.*]] = getelementptr inbounds i32, ptr [[BASE:%.*]], i64 [[IDX1:%.*]]
; CHECK-NEXT: [[P2_IDX:%.*]] = shl nsw i64 [[IDX2:%.*]], 2
-; CHECK-NEXT: [[P2:%.*]] = getelementptr inbounds i8, ptr [[P1]], i64 [[P2_IDX]]
-; CHECK-NEXT: [[P5:%.*]] = getelementptr inbounds i32, ptr [[BASE]], i64 [[IDX5:%.*]]
+; CHECK-NEXT: [[P2:%.*]] = getelementptr inbounds i8, ptr [[P1:%.*]], i64 [[P2_IDX]]
; CHECK-NEXT: [[P4_IDX:%.*]] = shl nsw i64 [[IDX4:%.*]], 2
-; CHECK-NEXT: [[P4:%.*]] = getelementptr inbounds i8, ptr [[P5]], i64 [[P4_IDX]]
-; CHECK-NEXT: call void @use(ptr [[P2]])
+; CHECK-NEXT: [[P3:%.*]] = getelementptr inbounds i8, ptr [[P2]], i64 [[P4_IDX]]
+; CHECK-NEXT: [[P3_IDX:%.*]] = shl nsw i64 [[IDX3:%.*]], 2
+; CHECK-NEXT: [[P4_IDX1:%.*]] = shl nsw i64 [[IDX7:%.*]], 2
+; CHECK-NEXT: [[P5:%.*]] = getelementptr inbounds i8, ptr [[P1]], i64 [[P4_IDX1]]
+; CHECK-NEXT: [[P5_IDX:%.*]] = shl nsw i64 [[IDX5:%.*]], 2
+; CHECK-NEXT: [[P4:%.*]] = getelementptr inbounds i8, ptr [[P5]], i64 [[P5_IDX]]
+; CHECK-NEXT: [[P6_IDX:%.*]] = shl nsw i64 [[IDX6:%.*]], 2
+; CHECK-NEXT: call void @use(ptr [[P3]])
; CHECK-NEXT: call void @use(ptr [[P4]])
-; CHECK-NEXT: [[P1_IDX1:%.*]] = add i64 [[IDX1]], [[IDX2]]
-; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[P1_IDX1]], [[IDX3:%.*]]
-; CHECK-NEXT: [[P4_IDX2:%.*]] = add i64 [[IDX5]], [[IDX4]]
-; CHECK-NEXT: [[TMP2:%.*]] = add i64 [[P4_IDX2]], [[IDX6:%.*]]
-; CHECK-NEXT: [[TMP3:%.*]] = sub i64 [[TMP1]], [[TMP2]]
-; CHECK-NEXT: [[GEPDIFF:%.*]] = shl i64 [[TMP3]], 2
+; CHECK-NEXT: [[TMP1:%.*]] = add nsw i64 [[P2_IDX]], [[P4_IDX]]
+; CHECK-NEXT: [[TMP2:%.*]] = add nsw i64 [[TMP1]], [[P3_IDX]]
+; CHECK-NEXT: [[TMP3:%.*]] = add nsw i64 [[P4_IDX1]], [[P5_IDX]]
+; CHECK-NEXT: [[TMP4:%.*]] = add nsw i64 [[TMP3]], [[P6_IDX]]
+; CHECK-NEXT: [[GEPDIFF:%.*]] = sub nsw i64 [[TMP2]], [[TMP4]]
; CHECK-NEXT: ret i64 [[GEPDIFF]]
;
%p1 = getelementptr inbounds i32, ptr %base, i64 %idx1
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