[musl] [PATCH] configure: prevent compilers from turning a * b + c into fma(a, b, c)

["Alex Rønne Petersen" <alex@alexrp.com>]

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