[llvm] Use compiler intrinsics to compute PowerOf2Ceil (PR #67580)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

<details>
<summary>Changes</summary>

Instead of calling NextPowerOf2, we can use compiler intrinsics to do this faster.

---
Full diff: https://github.com/llvm/llvm-project/pull/67580.diff


1 Files Affected:

- (modified) llvm/include/llvm/Support/MathExtras.h (+4) 


``````````diff
diff --git a/llvm/include/llvm/Support/MathExtras.h b/llvm/include/llvm/Support/MathExtras.h
index dc095941fdc8a9f..f592beee5ccffc2 100644
--- a/llvm/include/llvm/Support/MathExtras.h
+++ b/llvm/include/llvm/Support/MathExtras.h
@@ -361,7 +361,11 @@ constexpr inline uint64_t NextPowerOf2(uint64_t A) {
 inline uint64_t PowerOf2Ceil(uint64_t A) {
   if (!A)
     return 0;
+#if __has_builtin(__builtin_clzl)
+  return 1ULL << (64 - __builtin_clzl(A - 1));
+#else
   return NextPowerOf2(A - 1);
+#endif
 }
 
 /// Returns the next integer (mod 2**64) that is greater than or equal to

``````````

</details>


https://github.com/llvm/llvm-project/pull/67580