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Re: Enable EBX for x86 in 32bits PIC code


On Wed, Sep 24, 2014 at 03:20:44PM -0600, Jeff Law wrote:
> On 09/24/14 14:32, Ilya Enkovich wrote:
> >2014-09-24 19:27 GMT+04:00 Jeff Law <law@redhat.com>:
> >>On 09/24/14 00:56, Ilya Enkovich wrote:
> 
> >>>
> >>>After register allocation we have no idea where GOT address is and
> >>>therefore delegitimize_address target hook becomes less efficient and
> >>>cannot remove UNSPECs. That's what I see now when build GCC with patch
> >>>applied:
> >>
> >>In theory this shouldn't be too hard to fix.
> >>
> >>I haven't looked at the code, but it might be something looking explicitly
> >>for ebx by register #, or something similar.  Which case within
> >>delegitimize_address isn't firing as it should after your changes?
> >
> >It is the case I had to fix:
> >
> >@@ -14415,7 +14433,8 @@ ix86_delegitimize_address (rtx x)
> >          ...
> >          movl foo@GOTOFF(%ecx), %edx
> >          in which case we return (%ecx - %ebx) + foo.  */
> >-      if (pic_offset_table_rtx)
> >+      if (pic_offset_table_rtx
> >+         && (!reload_completed || !ix86_use_pseudo_pic_reg ()))
> >          result = gen_rtx_PLUS (Pmode, gen_rtx_MINUS (Pmode, copy_rtx (addend),
> >                                                      pic_offset_table_rtx),
> >                                result);
> >
> >Originally if there is a UNSPEC_GOTOFFSET but no EBX usage then we
> >just remove this UNSPEC and substract EBX value.  With pseudo PIC reg
> >we should use PIC register instead of EBX but it is unclear what to
> >use after register allocation.
> What's the RTL before & after allocation?  Feel free to just pass along the
> dump files for sum_r4 that you referenced in a prior message.

I wonder if during/after reload we just couldn't look at
ORIGINAL_REGNO of hard regs if ix86_use_pseudo_pic_reg.  Or is that
the other case, where you don't have any PIC register replacement around,
and want to subtract something?  Perhaps in that case we could just
subtract the value of _GLOBAL_OFFSET_TABLE_ symbol if we have nothing better
around.

	Jakub


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