[clang] Fix codegen of consteval functions returning an empty class. (PR #93115)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic created https://github.com/llvm/llvm-project/pull/93115

If a class is empty, don't store it to memory: the store might overwrite useful data.

(See also d60c3d08.)

Fixes #93040.

>From bdfcc729c309fc5b1092b67d7c3c803c852ae251 Mon Sep 17 00:00:00 2001
From: Eli Friedman <efriedma at quicinc.com>
Date: Wed, 22 May 2024 17:30:41 -0700
Subject: [PATCH] Fix codegen of consteval functions returning an empty class.

If a class is empty, don't store it to memory: the store might overwrite
useful data.

(See also d60c3d08.)

Fixes #93040.
---
 clang/lib/CodeGen/CGExprAgg.cpp           | 11 +++++++++++
 clang/test/CodeGenCXX/cxx2a-consteval.cpp | 24 ++++++++++++++++++++++-
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index bba00257fd4f0..b1638fa318270 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -135,6 +135,17 @@ class AggExprEmitter : public StmtVisitor<AggExprEmitter> {
     EnsureDest(E->getType());
 
     if (llvm::Value *Result = ConstantEmitter(CGF).tryEmitConstantExpr(E)) {
+      // An empty record can overlap other data (if declared with
+      // no_unique_address); omit the store for such types - as there is no
+      // actual data to store.
+      if (CGF.getLangOpts().CPlusPlus) {
+        if (const RecordType *RT = E->getType()->getAs<RecordType>()) {
+          CXXRecordDecl *Record = cast<CXXRecordDecl>(RT->getDecl());
+          if (Record->isEmpty())
+            return;
+        }
+      }
+
       Address StoreDest = Dest.getAddress();
       // The emitted value is guaranteed to have the same size as the
       // destination but can have a different type. Just do a bitcast in this
diff --git a/clang/test/CodeGenCXX/cxx2a-consteval.cpp b/clang/test/CodeGenCXX/cxx2a-consteval.cpp
index 075cab58358ab..5d5a62f9928fe 100644
--- a/clang/test/CodeGenCXX/cxx2a-consteval.cpp
+++ b/clang/test/CodeGenCXX/cxx2a-consteval.cpp
@@ -1,4 +1,3 @@
-// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py
 // RUN: %clang_cc1 -emit-llvm %s -std=c++2a -triple x86_64-unknown-linux-gnu -o %t.ll
 // RUN: FileCheck -check-prefix=EVAL -input-file=%t.ll %s
 // RUN: FileCheck -check-prefix=EVAL-STATIC -input-file=%t.ll %s
@@ -275,3 +274,26 @@ void f() {
     // EVAL-FN:     call void @_ZN7GH821542S3C2Ei
 }
 }
+
+namespace GH93040 {
+struct C { char c = 1; };
+struct Empty { consteval Empty() {} };
+struct Test : C, Empty {
+  [[no_unique_address]] Empty e;
+};
+
+void f() {
+  Test test;
+
+// Make sure we don't overwrite the initialization of c.
+
+// EVAL-FN-LABEL: define {{.*}} void @_ZN7GH930404TestC2Ev
+// EVAL-FN: entry:
+// EVAL-FN-NEXT:  [[THIS_ADDR:%.*]] = alloca ptr, align 8
+// EVAL-FN-NEXT:  store ptr {{.*}}, ptr [[THIS_ADDR]], align 8
+// EVAL-FN-NEXT:  [[THIS:%.*]] = load ptr, ptr [[THIS_ADDR]], align 8
+// EVAL-FN-NEXT:  call void @_ZN7GH930401CC2Ev(ptr noundef nonnull align 1 dereferenceable(1) [[THIS]])
+// EVAL-FN-NEXT:  %0 = getelementptr inbounds i8, ptr [[THIS]], i64 1
+// EVAL-FN-NEXT:  ret void
+}
+}