configure: disable TBAA optimization because most compilers are buggy

unlike most projects that use -fno-strict-aliasing, we aim to have all
sources respect the C language rules for effective type that make
type-based alias analysis optimizations possible. unfortunately, it
turns out that there are deep, and likely very difficult to fix, flaws
in the TBAA performed by GCC and likely other compilers, whereby this
kind of optimization can transform code that follows the rules
strictly in ways that will make it malfunction. see for example GCC
bugs 107107 and 107115, the latter of which also affects clang.

there are not presently any known instances of breakage due to wrong
type-based aliasing optimizations in our codebase. nonetheless, since
the transformations are unsound and could introduce breakage,
configure CFLAGS to build with -fno-strict-aliasing.

some casual analysis of the effects on codegen suggest that this is
unlikely to affect performance except possibly in the regex engine. in
general, we should probably prefer making better use of the restrict
keyword over relying on types to imply non-aliasing for optimization
purposes; doing so should be able to get back any performance that was
lost and more, should it turn out to matter (unlikely).

1 files changed, 8 insertions, 0 deletions

diff --git a/configure b/configure
index 6f5453f5..853bf05e 100755
--- a/configure
+++ b/configure
@@ -354,6 +354,14 @@ tryflag CFLAGS_C99FSE -fexcess-precision=standard \
 tryflag CFLAGS_C99FSE -frounding-math
 
 #
+# Semantically we want to insist that our sources follow the
+# C rules for type-based aliasing, but most if not all real-world
+# compilers are known or suspected to have critical bugs in their
+# type-based aliasing analysis. See for example GCC bug 107107.
+#
+tryflag CFLAGS_C99FSE -fno-strict-aliasing
+
+#
 # We may use the may_alias attribute if __GNUC__ is defined, so
 # if the compiler defines __GNUC__ but does not provide it,
 # it must be defined away as part of the CFLAGS.