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Re: [PATCH 3/3] Extend -falign-FOO=N to N[:M[:N2[:M2]]]


On Tue, Jul 03, 2018 at 10:53:20AM +0200, Martin Liška wrote:
> On 06/29/2018 09:04 PM, Jeff Law wrote:
> > I think this is fine for the trunk.
> > 
> > jeff
> 
> Thank you Jeff.
> 
> I found some issues when doing build of all targets (contrib/config-list.mk).
> I'll update patch and test that affected cross-compilers still produce same output.
> 
> However I noticed one ppc64 issue:
> 
> $ cat -n gcc/config/powerpcspe/powerpcspe.c
> 
>   5401        /* Set branch target alignment, if not optimizing for size.  */
>   5402        if (!optimize_size)
>   5403          {
>   5404            /* Cell wants to be aligned 8byte for dual issue.  Titan wants to be
>   5405               aligned 8byte to avoid misprediction by the branch predictor.  */
>   5406            if (rs6000_cpu == PROCESSOR_TITAN
>   5407                || rs6000_cpu == PROCESSOR_CELL)
>   5408              {
>   5409                if (align_functions <= 0)
>   5410                  align_functions = 8;
>   5411                if (align_jumps <= 0)
>   5412                  align_jumps = 8;
>   5413                if (align_loops <= 0)
>   5414                  align_loops = 8;
>   5415              }
>   5416            if (rs6000_align_branch_targets)
>   5417              {
>   5418                if (align_functions <= 0)
>   5419                  align_functions = 16;
>   5420                if (align_jumps <= 0)
>   5421                  align_jumps = 16;
>   5422                if (align_loops <= 0)
>   5423                  {
>   5424                    can_override_loop_align = 1;
>   5425                    align_loops = 16;
>   5426                  }
>   5427              }
>   5428            if (align_jumps_max_skip <= 0)
>   5429              align_jumps_max_skip = 15;
>   5430            if (align_loops_max_skip <= 0)
>   5431              align_loops_max_skip = 15;
> 
> Note that at line 5429 there's set of align_jumps_max_skip to 15 if not set by default.
> At line 5412 align_jumps is set to 8, and align_jumps_max_skip should be equal align_jumps - 1.
> That's a discrepancy. Segher can you please take a look?

This is powerpcspe, that's not mine.

But rs6000 has the same code, sure.  Why do you say "align_jumps_max_skip
should be equal align_jumps - 1"?  If that were true, why does it exist
at all?

toplev.c already has (in init_alignments):

  if (align_jumps_max_skip > align_jumps)
    align_jumps_max_skip = align_jumps - 1;

so why would targets duplicate that logic?  (The target override is called
before init_alignments).


Segher


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