Enable EBX for x86 in 32bits PIC code

[Uros Bizjak]

On Tue, Sep 23, 2014 at 3:54 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:

> Here is a patch which combines results of my and Vladimir's work on EBX enabling.
>
> It works OK for SPEC2000 and SPEC2006 on -Ofast + LTO.  It passes bootstrap but there are few new failures in make check.
>
> gcc.target/i386/pic-1.c fails because it doesn't expect we can use EBX in 32bit PIC mode
> gcc.target/i386/pr55458.c fails due to the same reason
> gcc.target/i386/pr23098.c fails because compiler fails to use float constant as an immediate and loads it from GOT instead
>
> Do we have the final decision about having a sompiler flag to control enabling of pseudo PIC register?  I think we should have a possibility to use fixed EBX at least until we make sure pseudo PIC doesn't harm debug info generation. If we have such option then gcc.target/i386/pic-1.c and gcc.target/i386/pr55458.c should be modified, otherwise these tests should be removed.

I think having this flag would be dangerous. In effect, this flag
would be a hidden -ffixed-bx, with unwanted consequences on asm code
that handles ebx. As an example, please see config/i386/cpuid.h - ATM,
we handle ebx in a special way when __PIC__ is defined. With your
patch, we will have to handle it in a special way when new flag is in
effect, which is impossible, unless another compiler-generated define
is emitted.

So, I vote to change PIC reg to a pseudo unconditionally and adjust
testsuite for all (expected) fall-out.

Uros.