[Bug rtl-optimization/48696] Horrible bitfield code generation on x86

[Jan Hubicka]

> Actually the 4.0.4 compiler is x86_64, the code with -m32.  The 4.0.3
> compiler is i586.
> 
>  /space/rguenther/install/gcc-4.0.3/libexec/gcc/i686-pc-linux-gnu/4.0.3/cc1
> -quiet -v t.c -quiet -dumpbase t.c -m32 -mtune=pentiumpro -auxbase t -O2
> -version -o t.s
> 
> 
> /space/rguenther/install/gcc-4.0.4/libexec/gcc/x86_64-unknown-linux-gnu/4.0.4/cc1
> -quiet -v t.c -quiet -dumpbase t.c -m32 -mtune=k8 -auxbase t -O2 -version -o
> t.s
> 
> but no -march/tune combination makes the bug vanish for the 4.0.4 compiler
> (maybe a HWI dependent "optimization")

We do have i386 flag to disable instruction choice that change memory access
size, it is X86_TUNE_MEMORY_MISMATCH_STALL and I remember myself adding code to
check that we don't do instruction selection that changes memory access size (i.e.
andl->andb promotion) unless when optimizing for size.

This code seems to've evaporated from backend, so I guess it needs re-adding ;)
It did not however work quite reliably since combiner actually does this kind of
transform himself from time to time and I did not get approval to get target
hook for that back then (in 2000 or so).

Honza