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Re: Inconsistent initialization for pic_offset_table_rtx?


2016-02-04 17:12 GMT+03:00 Bin.Cheng <amker.cheng@gmail.com>:
> Hi,
> I noticed that pic_offset_table_rtx is initialized twice in GCC.  Take
> x86_32 as an example.
> The first initialization is done in emit_init_regs, with below code:
>
>   pic_offset_table_rtx = NULL_RTX;
>   if ((unsigned) PIC_OFFSET_TABLE_REGNUM != INVALID_REGNUM)
>     pic_offset_table_rtx = gen_raw_REG (Pmode, PIC_OFFSET_TABLE_REGNUM);
>
> On x86_32 with pic, we have:
>
> (gdb) call debug_rtx(this_target_rtl->x_pic_offset_table_rtx)
> (reg:SI 3 bx)
>
> The second initialization is in expand_used_vars, with below code:
>
>   if (targetm.use_pseudo_pic_reg ())
>     pic_offset_table_rtx = gen_reg_rtx (Pmode);
>
> On x86_32 with pic, we have:
>
> (gdb) call debug_rtx(this_target_rtl->x_pic_offset_table_rtx)
> (reg:SI 87)
>
> So basically after expanding the first function, pic_offset_table_rtx
> is set to a pseudo register, rather than the one initialized in
> emit_init_regs.
>
> Also this causes inconsistent compilation for the first/rest functions
> in one compilation unit.
>
> A bug?

For i386 target PIC_OFFSET_TABLE_REGNUM actually checks
ix86_use_pseudo_pic_reg and is supposed to return INVALID_REGNUM
in case we use pseudo register for PIC. BUT we hit a case when PIC
code is generated for cost estimation via target hooks while performing
some GIMPLE pass. In this case we need to return some register to
generate PIC usage but we don't have any allocated. In this case we
return a hard register. We detect such situation by checking
pic_offset_table_rtx.

Thus if we use pseudo PIC register but pic_offset_table_rtx is not
initialized yet,
then PIC_OFFSET_TABLE_REGNUM returns a hard register.

So I suppose we may consider the first assignment as a bug.

Thanks,
Ilya

>
> Thanks,
> bin


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