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Message-ID: <20250529224200.921426-1-alex@alexrp.com>
Date: Fri, 30 May 2025 00:42:00 +0200
From: Alex Rønne Petersen <alex@...xrp.com>
To: musl@...ts.openwall.com
Cc: Alex Rønne Petersen <alex@...xrp.com>
Subject: [PATCH] configure: prevent gcc and clang from performing floating point contraction
The default setting for -ffp-contract is implementation-defined, so make
-ffp-contract=off explicit. This matters on clang in particular where
contraction is on by default even in standard C mode. Because of this, clang
will turn these expressions into llvm.fmuladd.* intrinsic calls in the
generated LLVM IR. This means we're completely at the mercy of the backend in
regards to whether those intrinsics get lowered to an fma() call or simply
fmul + fadd instructions. Similarly, optimization passes are allowed to perform
transformations on those intrinsic calls that fmul + fadd would not permit.
Older gcc versions had a similar issue and required -mno-fused-madd, hence set
this as well.
With zig cc -target arm-linux-musleabi, we've seen a case where clang turned
a * b + c into an fma() library call in musl's fma() implementation itself,
resulting in infinite recursion. This patch also fixes that.
---
configure | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/configure b/configure
index bc9fbe48..aa890101 100755
--- a/configure
+++ b/configure
@@ -355,6 +355,15 @@ tryflag CFLAGS_C99FSE -fexcess-precision=standard \
|| { test "$ARCH" = i386 && tryflag CFLAGS_C99FSE -ffloat-store ; }
tryflag CFLAGS_C99FSE -frounding-math
+#
+# Explicitly disable floating point contraction because the default
+# setting is implementation-defined, and clang is known to have it
+# on by default even in standard C mode. The same was true in older
+# gcc versions, hence -mno-fused-add as well.
+#
+tryflag CFLAGS_C99FSE -mno-fused-madd
+tryflag CFLAGS_C99FSE -ffp-contract=off
+
#
# Semantically we want to insist that our sources follow the
# C rules for type-based aliasing, but most if not all real-world
--
2.48.1
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