Re: [PATCH] Expand PIC calls without PLT with -fno-plt

[Jiong Wang <jiong.wang@arm.com>]

On 04/05/15 17:37, Alexander Monakov wrote:
> This patch introduces option -fno-plt that allows to expand calls that would
> go via PLT to load the address of the function immediately at call site (which
> introduces a GOT load).  Cover letter explains the motivation for this patch.
>
> New option documentation for invoke.texi is missing from the patch; if this is
> accepted I'll be happy to send a v2 with documentation added.
>
> 	* calls.c (prepare_call_address): Transform PLT call to GOT lookup and
> 	indirect call by forcing address into a pseudo with -fno-plt.
> 	* common.opt (flag_plt): New option.

Have done a quick experiment, -fno-plt doesn't work on AArch64.

it's because although this patch force the function address into register,
but the combine pass runs later combine it back as AArch64 have defined such
insn pattern.

For X86, it's not combined back. From the rtl dump, it's because the rtl 
pre pass
has moved the address load instruction into another basic block and 
combine pass
don't combine across basic blocks. Also, x86 backend has done some check 
on flag_plt
in the new added ix86_nopic_noplt_attribute_p which could help generate 
correct insns.

What I can think of the fix on AArch64 is by restricting the call symbol 
under
"flag_plt == true" only, so that call via register can't be combined 
into call
symbol direct,

Or better to prohibit combine pass for such combining? as the generic 
fix on combine may
fix other broken targets.

Thoughts?

Regards,
Jiong