[clang] [clang][CodeGen] fix UB in aarch64 bfloat16 scalar conversion (PR #89062)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: None (nihui)

<details>
<summary>Changes</summary>

do not bitcast 16bit `bfloat16` to 32bit `int32_t` directly
bitcast to `int16_t`, and then upcast to `int32_t`

Fix ASAN runtime error when calling vcvtah_f32_bf16
`==21842==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x007fda1dd063 at pc 0x005c0361c234 bp 0x007fda1dd030 sp 0x007fda1dd028
`

without patch
```c
__ai __attribute__((target("bf16"))) float32_t vcvtah_f32_bf16(bfloat16_t __p0) {
  float32_t __ret;
bfloat16_t __reint = __p0;
int32_t __reint1 = *(int32_t *) &__reint << 16;
  __ret = *(float32_t *) &__reint1;
  return __ret;
}
```

with this patch
```c
__ai __attribute__((target("bf16"))) float32_t vcvtah_f32_bf16(bfloat16_t __p0) {
  float32_t __ret;
bfloat16_t __reint = __p0;
int32_t __reint1 = (int32_t)(*(int16_t *) &__reint) << 16;
  __ret = *(float32_t *) &__reint1;
  return __ret;
}
```

fix issue https://github.com/llvm/llvm-project/issues/61983

---
Full diff: https://github.com/llvm/llvm-project/pull/89062.diff


1 Files Affected:

- (modified) clang/include/clang/Basic/arm_neon.td (+1-1) 


``````````diff
diff --git a/clang/include/clang/Basic/arm_neon.td b/clang/include/clang/Basic/arm_neon.td
index 6d655c39360d3b..6390ba3f9fe5e5 100644
--- a/clang/include/clang/Basic/arm_neon.td
+++ b/clang/include/clang/Basic/arm_neon.td
@@ -275,7 +275,7 @@ def OP_VCVT_BF16_F32_HI_A32
                            (call "vget_low", $p0))>;
 
 def OP_CVT_F32_BF16
-    : Op<(bitcast "R", (op "<<", (bitcast "int32_t", $p0),
+    : Op<(bitcast "R", (op "<<", (cast "int32_t", (bitcast "int16_t", $p0)),
                                  (literal "int32_t", "16")))>;
 
 //===----------------------------------------------------------------------===//

``````````

</details>


https://github.com/llvm/llvm-project/pull/89062