[clang] [clang][CodeGen] Fix _BitInt return value miscompile (PR #179456)

[via cfe-commits]

https://github.com/mugiwaraluffy56 updated https://github.com/llvm/llvm-project/pull/179456

>From 6bac61c7257f58de90e8e44bfdad8e44e41840e1 Mon Sep 17 00:00:00 2001
From: puneeth_aditya_5656 <myakampuneeth at gmail.com>
Date: Tue, 3 Feb 2026 18:59:10 +0530
Subject: [PATCH] [clang][CodeGen] Fix _BitInt return value miscompile

Use CreateMemTemp instead of CreateIRTemp for the return value alloca
to ensure proper memory representation for types like _BitInt that have
different IR and memory types.

For _BitInt(121), ConvertType returns i121 but ConvertTypeForMem returns
i128. When the return value is coerced to {i64, i64} for the ABI, storing
i121 and loading {i64, i64} would leave undefined bits in the result.
Using the memory type (i128) ensures all 128 bits are properly initialized.

Fixes #179448
---
 clang/lib/CodeGen/CodeGenFunction.cpp | 10 +++++++++-
 clang/test/CodeGen/ext-int.c          |  9 +++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index fbae2433ec278..b208fe0ecc550 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -1248,7 +1248,15 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
     ReturnValue = Address(Addr, ConvertType(RetTy),
                           CGM.getNaturalTypeAlignment(RetTy), KnownNonNull);
   } else {
-    ReturnValue = CreateIRTemp(RetTy, "retval");
+    // For CoerceAndExpand returns where IR and memory types differ
+    // (e.g., _BitInt(121) has IR type i121 but memory type i128),
+    // use CreateMemTemp so the alloca has the memory type. This ensures
+    // proper coercion when the epilog reads it as the coerced type.
+    if (CurFnInfo->getReturnInfo().getKind() == ABIArgInfo::CoerceAndExpand &&
+        ConvertType(RetTy) != ConvertTypeForMem(RetTy))
+      ReturnValue = CreateMemTemp(RetTy, "retval");
+    else
+      ReturnValue = CreateIRTemp(RetTy, "retval");
 
     // Tell the epilog emitter to autorelease the result.  We do this
     // now so that various specialized functions can suppress it
diff --git a/clang/test/CodeGen/ext-int.c b/clang/test/CodeGen/ext-int.c
index a12b11adbf00d..319bbe86558e8 100644
--- a/clang/test/CodeGen/ext-int.c
+++ b/clang/test/CodeGen/ext-int.c
@@ -252,4 +252,13 @@ void bitField() {
   // LIN64: store i64 %bf.set4, ptr %s1, align 8
 }
 
+// GH#179448: Ensure the return value alloca uses the memory type (i128)
+// instead of the IR type (i121) to avoid miscompilation when coercing
+// the return value to {i64, i64}.
+// LIN64: define {{.*}}{ i64, i64 } @returnBitInt121(i64 {{.*}}, i64 {{.*}})
+// LIN64: %retval = alloca i128, align 8
+_BitInt(121) returnBitInt121(_BitInt(121) a) {
+  return a;
+}
+
 #endif