[llvm-branch-commits] [llvm] release/20.x: [DSE] Don't use initializes on byval argument (#126259) (PR #126493)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/126493

Backport 2d31a12dbe2339d20844ede70cbb54dbaf4ceea9

Requested by: @nikic

>From 50830ff14060ff6ff8a3c6062aff2a5b835c4d33 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Mon, 10 Feb 2025 10:34:03 +0100
Subject: [PATCH] [DSE] Don't use initializes on byval argument (#126259)

There are two ways we can fix this problem, depending on how the
semantics of byval and initializes should interact:

* Don't infer initializes on byval arguments. initializes on byval
refers to the original caller memory (or having both attributes is made
a verifier error).
* Infer initializes on byval, but don't use it in DSE. initializes on
byval refers to the callee copy. This matches the semantics of readonly
on byval. This is slightly more powerful, for example, we could do a
backend optimization where byval + initializes will allocate the full
size of byval on the stack but not copy over the parts covered by
initializes.

I went with the second variant here, skipping byval + initializes in DSE
(FunctionAttrs already doesn't propagate initializes past byval). I'm
open to going in the other direction though.

Fixes https://github.com/llvm/llvm-project/issues/126181.

(cherry picked from commit 2d31a12dbe2339d20844ede70cbb54dbaf4ceea9)
---
 llvm/docs/LangRef.rst                              |  4 ++++
 .../lib/Transforms/Scalar/DeadStoreElimination.cpp |  4 +++-
 .../DeadStoreElimination/inter-procedural.ll       | 14 ++++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

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
+}