[gambit-list] Getting rid of the -fwrapv flag in gcc
lucier at math.purdue.edu
Sun Apr 15 18:39:22 EDT 2007
I've been thinking about how to get the C code generated by gsc to
conform to the ISO C standard. Beyond being just a "good thing",
this would allow us to get rid of the -fwrapv and -fno-strict-
aliasing flags in gcc, which would also enable some further compiler
optimizations. (Well, getting rid of -fwrapv enables some
optimizations and disables others.)
I think the only places in the gambit back-end where we assume that
signed arithmetic wraps on overflow are in the macros FIXADDP,
FIXMULP, and FIXSUBP in gambit.h. I'm no expert on signed vs.
unsigned arithmetic in C, but I think that simply casting the
arguments of the possibly overflowing operations to unsigned, and
then casting the result back to signed, will, in the end, give us the
same result but will be standard-conforming.
So I offer this (lightly tested, but it did pass "make check") patch
for gambit.h that does precisely this. This should allow us to omit
the -fwrapv flag for gcc. I don't have any performance figures for
gsc compiled without -fwrapv, but the binary is a bit smaller:
[brad:~/programs/gambc-4.0b22] lucier% ll gsc/gsc gsc-comp
-rwxr-xr-x 1 lucier lucier 5544476 Apr 6 19:11 gsc-comp*
-rwxr-xr-x 1 lucier lucier 5540296 Apr 15 17:58 gsc/gsc*
The patch also has an extra guard to limit the use of
__builtin_expect to versions of gcc >= 3.0.
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