release/18.x: [InstCombine] Fold gep of exact unsigned division (#82334)

[llvmbot]

Backport ec2c770 26d4afc

Requested by: @nikic

[llvmbot]

@dtcxzyw What do you think about merging this PR to the release branch?

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: None (llvmbot)

Changes

Backport ec2c770 26d4afc

Requested by: @nikic

---

Full diff: https://github.com/llvm/llvm-project/pull/82347.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+3-3)
• (modified) llvm/test/Transforms/InstCombine/getelementptr.ll (+54)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 249f4a7710e046..5d207dcfd18dd4 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2594,10 +2594,10 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
         Value *V;
         if ((has_single_bit(TyAllocSize) &&
              match(GEP.getOperand(1),
-                   m_Exact(m_AShr(m_Value(V),
-                                  m_SpecificInt(countr_zero(TyAllocSize)))))) ||
+                   m_Exact(m_Shr(m_Value(V),
+                                 m_SpecificInt(countr_zero(TyAllocSize)))))) ||
             match(GEP.getOperand(1),
-                  m_Exact(m_SDiv(m_Value(V), m_SpecificInt(TyAllocSize))))) {
+                  m_Exact(m_IDiv(m_Value(V), m_SpecificInt(TyAllocSize))))) {
           GetElementPtrInst *NewGEP = GetElementPtrInst::Create(
               Builder.getInt8Ty(), GEP.getPointerOperand(), V);
           NewGEP->setIsInBounds(GEP.isInBounds());
diff --git a/llvm/test/Transforms/InstCombine/getelementptr.ll b/llvm/test/Transforms/InstCombine/getelementptr.ll
index 642c3eb2a0e41b..c90b6c9fb29592 100644
--- a/llvm/test/Transforms/InstCombine/getelementptr.ll
+++ b/llvm/test/Transforms/InstCombine/getelementptr.ll
@@ -116,6 +116,7 @@ define void @test_overaligned_vec(i8 %B) {
 ; CHECK-LABEL: @test_overaligned_vec(
 ; CHECK-NEXT:    store i8 [[B:%.*]], ptr getelementptr inbounds ([10 x i8], ptr @Global, i64 0, i64 2), align 1
 ; CHECK-NEXT:    ret void
+;
   %A = getelementptr <2 x half>, ptr @Global, i64 0, i64 1
   store i8 %B, ptr %A
   ret void
@@ -1473,6 +1474,16 @@ define ptr @gep_sdiv(ptr %p, i64 %off) {
   ret ptr %ptr
 }
 
+define ptr @gep_udiv(ptr %p, i64 %off) {
+; CHECK-LABEL: @gep_udiv(
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr i8, ptr [[P:%.*]], i64 [[OFF:%.*]]
+; CHECK-NEXT:    ret ptr [[PTR]]
+;
+  %index = udiv exact i64 %off, 7
+  %ptr = getelementptr %struct.C, ptr %p, i64 %index
+  ret ptr %ptr
+}
+
 define <2 x ptr> @gep_sdiv_vec(<2 x ptr> %p, <2 x i64> %off) {
 ; CHECK-LABEL: @gep_sdiv_vec(
 ; CHECK-NEXT:    [[PTR:%.*]] = getelementptr i8, <2 x ptr> [[P:%.*]], <2 x i64> [[OFF:%.*]]
@@ -1503,6 +1514,16 @@ define ptr @gep_ashr(ptr %p, i64 %off) {
   ret ptr %ptr
 }
 
+define ptr @gep_lshr(ptr %p, i64 %off) {
+; CHECK-LABEL: @gep_lshr(
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr i8, ptr [[P:%.*]], i64 [[OFF:%.*]]
+; CHECK-NEXT:    ret ptr [[PTR]]
+;
+  %index = lshr exact i64 %off, 2
+  %ptr = getelementptr i32, ptr %p, i64 %index
+  ret ptr %ptr
+}
+
 ; Negative tests
 
 define ptr @gep_i8(ptr %p, i64 %off) {
@@ -1525,6 +1546,17 @@ define ptr @gep_sdiv_mismatched_size(ptr %p, i64 %off) {
   ret ptr %ptr
 }
 
+define ptr @gep_udiv_mismatched_size(ptr %p, i64 %off) {
+; CHECK-LABEL: @gep_udiv_mismatched_size(
+; CHECK-NEXT:    [[INDEX:%.*]] = udiv exact i64 [[OFF:%.*]], 20
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr [[STRUCT_C:%.*]], ptr [[P:%.*]], i64 [[INDEX]]
+; CHECK-NEXT:    ret ptr [[PTR]]
+;
+  %index = udiv exact i64 %off, 20
+  %ptr = getelementptr %struct.C, ptr %p, i64 %index
+  ret ptr %ptr
+}
+
 define ptr @gep_sdiv_without_exact(ptr %p, i64 %off) {
 ; CHECK-LABEL: @gep_sdiv_without_exact(
 ; CHECK-NEXT:    [[INDEX:%.*]] = sdiv i64 [[OFF:%.*]], 7
@@ -1536,6 +1568,17 @@ define ptr @gep_sdiv_without_exact(ptr %p, i64 %off) {
   ret ptr %ptr
 }
 
+define ptr @gep_udiv_without_exact(ptr %p, i64 %off) {
+; CHECK-LABEL: @gep_udiv_without_exact(
+; CHECK-NEXT:    [[INDEX:%.*]] = udiv i64 [[OFF:%.*]], 7
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr [[STRUCT_C:%.*]], ptr [[P:%.*]], i64 [[INDEX]]
+; CHECK-NEXT:    ret ptr [[PTR]]
+;
+  %index = udiv i64 %off, 7
+  %ptr = getelementptr %struct.C, ptr %p, i64 %index
+  ret ptr %ptr
+}
+
 define ptr @gep_ashr_without_exact(ptr %p, i64 %off) {
 ; CHECK-LABEL: @gep_ashr_without_exact(
 ; CHECK-NEXT:    [[INDEX:%.*]] = ashr i64 [[OFF:%.*]], 2
@@ -1547,6 +1590,17 @@ define ptr @gep_ashr_without_exact(ptr %p, i64 %off) {
   ret ptr %ptr
 }
 
+define ptr @gep_lshr_without_exact(ptr %p, i64 %off) {
+; CHECK-LABEL: @gep_lshr_without_exact(
+; CHECK-NEXT:    [[INDEX:%.*]] = lshr i64 [[OFF:%.*]], 2
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[INDEX]]
+; CHECK-NEXT:    ret ptr [[PTR]]
+;
+  %index = lshr i64 %off, 2
+  %ptr = getelementptr i32, ptr %p, i64 %index
+  ret ptr %ptr
+}
+
 define i1 @test_only_used_by_icmp(ptr %a, ptr %b, ptr %c) {
 ; CHECK-LABEL: @test_only_used_by_icmp(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[B:%.*]], [[C:%.*]]

[nikic]

Submitting this for backport, because it mitigates an LLVM 18 optimization regression encountered in Rust. The regression is really in a different area, but I think this is the least intrusive way to mitigate it.

[dtcxzyw]

LGTM if CI is happy.