release/20.x: [DSE] Don't use initializes on byval argument (#126259)

[llvmbot]

Backport 2d31a12

Requested by: @nikic

[llvmbot]

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

[llvmbot]

@llvm/pr-subscribers-llvm-ir

@llvm/pr-subscribers-llvm-transforms

Author: None (llvmbot)

Changes

Backport 2d31a12

Requested by: @nikic

---

Full diff: https://github.com/llvm/llvm-project/pull/126493.diff

3 Files Affected:

• (modified) llvm/docs/LangRef.rst (+4)
• (modified) llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp (+3-1)
• (modified) llvm/test/Transforms/DeadStoreElimination/inter-procedural.ll (+14)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index d004ced9dff1468..e002195cb7ed588 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1725,6 +1725,10 @@ Currently, only the following parameter attributes are defined:
     and negative values are allowed in case the argument points partway into
     an allocation. An empty list is not allowed.
 
+    On a ``byval`` argument, ``initializes`` refers to the given parts of the
+    callee copy being overwritten. A ``byval`` callee can never initialize the
+    original caller memory passed to the ``byval`` argument.
+
 ``dead_on_unwind``
     At a high level, this attribute indicates that the pointer argument is dead
     if the call unwinds, in the sense that the caller will not depend on the
diff --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
index 13f3de07c3c44d0..0fdc3354753b183 100644
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -2281,7 +2281,9 @@ DSEState::getInitializesArgMemLoc(const Instruction *I) {
   for (unsigned Idx = 0, Count = CB->arg_size(); Idx < Count; ++Idx) {
     ConstantRangeList Inits;
     Attribute InitializesAttr = CB->getParamAttr(Idx, Attribute::Initializes);
-    if (InitializesAttr.isValid())
+    // initializes on byval arguments refers to the callee copy, not the
+    // original memory the caller passed in.
+    if (InitializesAttr.isValid() && !CB->isByValArgument(Idx))
       Inits = InitializesAttr.getValueAsConstantRangeList();
 
     Value *CurArg = CB->getArgOperand(Idx);
diff --git a/llvm/test/Transforms/DeadStoreElimination/inter-procedural.ll b/llvm/test/Transforms/DeadStoreElimination/inter-procedural.ll
index e590c5bf4004afd..5f8ab56c22754d4 100644
--- a/llvm/test/Transforms/DeadStoreElimination/inter-procedural.ll
+++ b/llvm/test/Transforms/DeadStoreElimination/inter-procedural.ll
@@ -338,3 +338,17 @@ define i16 @global_var_alias() {
   ret i16 %l
 }
 
+declare void @byval_fn(ptr byval(i32) initializes((0, 4)) %am)
+
+define void @test_byval() {
+; CHECK-LABEL: @test_byval(
+; CHECK-NEXT:    [[A:%.*]] = alloca i32, align 4
+; CHECK-NEXT:    store i32 0, ptr [[A]], align 4
+; CHECK-NEXT:    call void @byval_fn(ptr [[A]])
+; CHECK-NEXT:    ret void
+;
+  %a = alloca i32
+  store i32 0, ptr %a
+  call void @byval_fn(ptr %a)
+  ret void
+}

[github-actions]

@nikic (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.