[libc] Suppress GCC loop optimization warning

[jhuber6]

Summary:
This appears to be a false positive on some versions of GCC.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Joseph Huber (jhuber6)

Changes

Summary:
This appears to be a false positive on some versions of GCC.

---

Full diff: https://github.com/llvm/llvm-project/pull/132458.diff

1 Files Affected:

• (modified) libc/src/__support/big_int.h (+8-2)

diff --git a/libc/src/__support/big_int.h b/libc/src/__support/big_int.h
index 85db31d01399a..2a4a72e30b352 100644
--- a/libc/src/__support/big_int.h
+++ b/libc/src/__support/big_int.h
@@ -790,11 +790,17 @@ struct BigInt {
       remainder[pos] = rem;
     }
 
+    // GCC currently emits a false-positive warning on this for some compilers.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Waggressive-loop-optimizations"
+
     // Set the remaining lower bits of the remainder.
     for (; pos > 0; --pos) {
       remainder[pos - 1] = val[pos - 1];
     }
 
+#pragma GCC diagnostic pop
+
     *this = quotient;
     return remainder;
   }
@@ -851,8 +857,8 @@ struct BigInt {
       result[i] = lhs[i] OP rhs[i];                                            \
     return result;                                                             \
   }                                                                            \
-  LIBC_INLINE friend constexpr BigInt operator OP##=(BigInt &lhs,              \
-                                                     const BigInt &rhs) {      \
+  LIBC_INLINE friend constexpr BigInt operator OP## =                          \
+      (BigInt & lhs, const BigInt &rhs) {                                      \
     for (size_t i = 0; i < WORD_COUNT; ++i)                                    \
       lhs[i] OP## = rhs[i];                                                    \
     return lhs;                                                                \

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 3757ecf5f16c0d9b8cbfc1c9d41965df537d43e6 7cdeffa47c66a966a5c53ba863f763f2f6a01107 --extensions h -- libc/src/__support/big_int.h

View the diff from clang-format here.

diff --git a/libc/src/__support/big_int.h b/libc/src/__support/big_int.h
index 25dddfef9c..c4b2345c49 100644
--- a/libc/src/__support/big_int.h
+++ b/libc/src/__support/big_int.h
@@ -861,8 +861,8 @@ public:
       result[i] = lhs[i] OP rhs[i];                                            \
     return result;                                                             \
   }                                                                            \
-  LIBC_INLINE friend constexpr BigInt operator OP## =                          \
-      (BigInt & lhs, const BigInt &rhs) {                                      \
+  LIBC_INLINE friend constexpr BigInt operator OP##=(BigInt &lhs,              \
+                                                     const BigInt &rhs) {      \
     for (size_t i = 0; i < WORD_COUNT; ++i)                                    \
       lhs[i] OP## = rhs[i];                                                    \
     return lhs;                                                                \

[lntue]

Look like you need to guard this for gcc only:

/home/runner/work/llvm-project/llvm-project/libc/src/__support/big_int.h:796:32: error: unknown warning group '-Waggressive-loop-optimizations', ignored [-Werror,-Wunknown-warning-option]
  796 | #pragma GCC diagnostic ignored "-Waggressive-loop-optimizations"