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Message-Id: <20240828152826.826990-1-alex@alexrp.com>
Date: Wed, 28 Aug 2024 17:28:26 +0200
From: Alex Rønne Petersen <alex@...xrp.com>
To: musl@...ts.openwall.com
Cc: Alex Rønne Petersen <alex@...xrp.com>
Subject: [PATCH] configure: prevent compilers from turning a * b + c into fma(a, b, c)
I've seen Clang do this for expressions in the fma() implementation itself,
which of course led to infinite recursion. This happened when targeting
arm-linux-musleabi with full soft float mode and -march=armv8-a. I imagine
it's possible for GCC to do similar silliness.
Work around this by passing -ffp-contract=off for Clang and -mno-fused-madd
for GCC. This matches what glibc's configure.ac does, FWIW.
---
configure | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/configure b/configure
index bc9fbe48..7028793f 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
+#
+# Prevent the compiler from turning a * b + c into an fma() call.
+# Clang at least has been known to do this in the implementation of
+# fma() itself when targeting arm-linux-musleabi and armv8-a, causing
+# infinite recursion.
+#
+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
--
2.40.1
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