[clang][CodeGen] fix UB in aarch64 bfloat16 scalar conversion

[nihui]

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

__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

__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 #61983

[llvmbot]

@llvm/pr-subscribers-clang

Author: None (nihui)

Changes

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

__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

__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 #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 --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")))>;
 
 //===----------------------------------------------------------------------===//

[nihui]

@ostannard hi, would you please review my fix :)

[ostannard]

LGTM, thanks.

[nihui]

LGTM, thanks.

And please merge this fix for me, I have no commit access :)