[llvm] [ArgPromotion] Remove incorrect TranspBlocks set for loads. (PR #84835)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

The TranspBlocks set was used to cache aliasing decision for all processed loads in the parent loop. This is incorrect, because each load can access a different location, which means one load not being modified in a block doesn't translate to another load not being modified in the same block.

All loads access the same underlying object, so we could perhaps use a location without size for all loads and retain the cache, but that would mean we loose precision.

For now, just drop the cache.

Fixes https://github.com/llvm/llvm-project/issues/84807

---
Full diff: https://github.com/llvm/llvm-project/pull/84835.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/IPO/ArgumentPromotion.cpp (+1-5) 
- (modified) llvm/test/Transforms/ArgumentPromotion/aliasing-and-non-aliasing-loads-with-clobber.ll (+5-5) 


``````````diff
diff --git a/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp b/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
index e89ec353487eef..3aa8ea3f514713 100644
--- a/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
+++ b/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
@@ -653,10 +653,6 @@ static bool findArgParts(Argument *Arg, const DataLayout &DL, AAResults &AAR,
   // check to see if the pointer is guaranteed to not be modified from entry of
   // the function to each of the load instructions.
 
-  // Because there could be several/many load instructions, remember which
-  // blocks we know to be transparent to the load.
-  df_iterator_default_set<BasicBlock *, 16> TranspBlocks;
-
   for (LoadInst *Load : Loads) {
     // Check to see if the load is invalidated from the start of the block to
     // the load itself.
@@ -670,7 +666,7 @@ static bool findArgParts(Argument *Arg, const DataLayout &DL, AAResults &AAR,
     // To do this, we perform a depth first search on the inverse CFG from the
     // loading block.
     for (BasicBlock *P : predecessors(BB)) {
-      for (BasicBlock *TranspBB : inverse_depth_first_ext(P, TranspBlocks))
+      for (BasicBlock *TranspBB : inverse_depth_first(P))
         if (AAR.canBasicBlockModify(*TranspBB, Loc))
           return false;
     }
diff --git a/llvm/test/Transforms/ArgumentPromotion/aliasing-and-non-aliasing-loads-with-clobber.ll b/llvm/test/Transforms/ArgumentPromotion/aliasing-and-non-aliasing-loads-with-clobber.ll
index 69385a7ea51a74..1c771d550b2192 100644
--- a/llvm/test/Transforms/ArgumentPromotion/aliasing-and-non-aliasing-loads-with-clobber.ll
+++ b/llvm/test/Transforms/ArgumentPromotion/aliasing-and-non-aliasing-loads-with-clobber.ll
@@ -14,10 +14,7 @@ define i32 @caller1(i1 %c) {
 ; CHECK-LABEL: define i32 @caller1(
 ; CHECK-SAME: i1 [[C:%.*]]) {
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[F_VAL:%.*]] = load i16, ptr @f, align 8
-; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i8, ptr @f, i64 8
-; CHECK-NEXT:    [[F_VAL1:%.*]] = load i64, ptr [[TMP0]], align 8
-; CHECK-NEXT:    call void @callee1(i16 [[F_VAL]], i64 [[F_VAL1]], i1 [[C]])
+; CHECK-NEXT:    call void @callee1(ptr noundef nonnull @f, i1 [[C]])
 ; CHECK-NEXT:    ret i32 0
 ;
 entry:
@@ -27,13 +24,16 @@ entry:
 
 define internal void @callee1(ptr nocapture noundef readonly %q, i1 %c) {
 ; CHECK-LABEL: define internal void @callee1(
-; CHECK-SAME: i16 [[Q_0_VAL:%.*]], i64 [[Q_8_VAL:%.*]], i1 [[C:%.*]]) {
+; CHECK-SAME: ptr nocapture noundef readonly [[Q:%.*]], i1 [[C:%.*]]) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 [[C]], label [[THEN:%.*]], label [[EXIT:%.*]]
 ; CHECK:       then:
 ; CHECK-NEXT:    store i16 123, ptr @f, align 8
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
+; CHECK-NEXT:    [[Q_0_VAL:%.*]] = load i16, ptr [[Q]], align 8
+; CHECK-NEXT:    [[GEP_8:%.*]] = getelementptr inbounds i8, ptr [[Q]], i64 8
+; CHECK-NEXT:    [[Q_8_VAL:%.*]] = load i64, ptr [[GEP_8]], align 8
 ; CHECK-NEXT:    call void @use(i16 [[Q_0_VAL]], i64 [[Q_8_VAL]])
 ; CHECK-NEXT:    ret void
 ;

``````````

</details>


https://github.com/llvm/llvm-project/pull/84835