[Transforms] Preserve inbounds attribute of transformed GEPs when flattening loops

[AtariDreams]

When flattening the loop, if the GEP was inbound, it should stay inbound, because the only thing that changed is how the pointers are calculated, not the elements being accessed.

Proof: https://alive2.llvm.org/ce/z/dApMpQ

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: AtariDreams (AtariDreams)

Changes

When flattening the loop, if the GEP was inbound, it should stay inbound, because the only thing that changed is how the pointers are calculated, not the elements being accessed.

---

Full diff: https://github.com/llvm/llvm-project/pull/86961.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/LoopFlatten.cpp (+3-2)
• (modified) llvm/test/Transforms/LoopFlatten/loop-flatten-gep.ll (+3-3)

diff --git a/llvm/lib/Transforms/Scalar/LoopFlatten.cpp b/llvm/lib/Transforms/Scalar/LoopFlatten.cpp
index 0e9cf328f149be..9316333aa2dd6d 100644
--- a/llvm/lib/Transforms/Scalar/LoopFlatten.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopFlatten.cpp
@@ -808,8 +808,9 @@ static bool DoFlattenLoopPair(FlattenInfo &FI, DominatorTree *DT, LoopInfo *LI,
       // we need to insert the new GEP where the old GEP was.
       if (!DT->dominates(Base, &*Builder.GetInsertPoint()))
         Builder.SetInsertPoint(cast<Instruction>(V));
-      OuterValue = Builder.CreateGEP(GEP->getSourceElementType(), Base,
-                                     OuterValue, "flatten." + V->getName());
+      OuterValue =
+          Builder.CreateGEP(GEP->getSourceElementType(), Base, OuterValue,
+                            "flatten." + V->getName(), GEP->isInBounds());
     }
 
     LLVM_DEBUG(dbgs() << "Replacing: "; V->dump(); dbgs() << "with:      ";
diff --git a/llvm/test/Transforms/LoopFlatten/loop-flatten-gep.ll b/llvm/test/Transforms/LoopFlatten/loop-flatten-gep.ll
index f4b8ea97237fe6..e30001670b1e95 100644
--- a/llvm/test/Transforms/LoopFlatten/loop-flatten-gep.ll
+++ b/llvm/test/Transforms/LoopFlatten/loop-flatten-gep.ll
@@ -15,7 +15,7 @@ for.outer.preheader:
   br label %for.inner.preheader
 
 ; CHECK-LABEL: for.inner.preheader:
-; CHECK: %flatten.arrayidx = getelementptr i32, ptr %A, i32 %i
+; CHECK: %flatten.arrayidx = getelementptr inbounds i32, ptr %A, i32 %i
 for.inner.preheader:
   %i = phi i32 [ 0, %for.outer.preheader ], [ %inc2, %for.outer ]
   br label %for.inner
@@ -61,13 +61,13 @@ for.outer.preheader:
   br label %for.inner.preheader
 
 ; CHECK-LABEL: for.inner.preheader:
-; CHECK-NOT: getelementptr i32, ptr %ptr, i32 %i
+; CHECK-NOT: getelementptr inbounds i32, ptr %ptr, i32 %i
 for.inner.preheader:
   %i = phi i32 [ 0, %for.outer.preheader ], [ %inc2, %for.outer ]
   br label %for.inner
 
 ; CHECK-LABEL: for.inner:
-; CHECK: %flatten.arrayidx = getelementptr i32, ptr %ptr, i32 %i
+; CHECK: %flatten.arrayidx = getelementptr inbounds i32, ptr %ptr, i32 %i
 ; CHECK: store i32 0, ptr %flatten.arrayidx, align 4
 ; CHECK: br label %for.outer
 for.inner:

[AtariDreams]

Proof: https://alive2.llvm.org/ce/z/dApMpQ

[AtariDreams]

@fhahn What do you think about this?

[AtariDreams]

@john-brawn-arm What do you think about this?

[john-brawn-arm]

Looks good to me.

[AtariDreams]

@dtcxzyw What do you think?

[AtariDreams]

Looks good to me.

Thank you!