Re: LRA vs reload on powerpc: 2 extra FAILs that are actually improvements?

[David Edelsohn <dje.gcc@gmail.com>]

Hi, Steven

Thanks for investigating this. This presumably was the reason that
Vlad changed the constraint modifier for that pattern in his patch for
LRA.  I don't think that using memory is an improvement, but Mike is
the best person to comment.

Thanks, David

On Sat, Nov 2, 2013 at 6:48 PM, Steven Bosscher <stevenb.gcc@gmail.com> wrote:
> Hello,
>
> Today's powerpc64-linux gcc has 2 extra failures with -mlra vs. reload
> (i.e. svn unpatched).
>
> (I'm excluding guality failure differences here because there are too
> many of them that seem to fail at random after minimal changes
> anywhere in the compiler...).
>
> Test results are posted here:
> reload: http://gcc.gnu.org/ml/gcc-testresults/2013-11/msg00128.html
> lra: http://gcc.gnu.org/ml/gcc-testresults/2013-11/msg00129.html
>
> The new failures and total score is as follows (+=lra, -=reload):
> +FAIL: gcc.target/powerpc/pr53199.c scan-assembler-times stwbrx 6
> +FAIL: gcc.target/powerpc/pr58330.c scan-assembler-not stwbrx
>
>                 === gcc Summary ===
>
> -# of expected passes           97887
> -# of unexpected failures       536
> +# of expected passes           97903
> +# of unexpected failures       538
>  # of unexpected successes      38
>  # of expected failures         244
> -# of unsupported tests         1910
> +# of unsupported tests         1892
>
>
> The failure of pr53199.c is because of different instruction selection
> for bswap. Test case is reduced to just one function:
>
> /* { dg-options "-O2 -mcpu=power6 -mavoid-indexed-addresses" } */
> long long
> reg_reverse (long long x)
> {
>   return __builtin_bswap64 (x);
> }
>
> Reload left vs. LRA right:
> reg_reverse:                           reg_reverse:
>         srdi 8,3,32                  |         addi 8,1,-16
>         rlwinm 7,3,8,0xffffffff      |         srdi 10,3,32
>         rlwinm 9,8,8,0xffffffff      |         addi 9,8,4
>         rlwimi 7,3,24,0,7            |         stwbrx 3,0,8
>         rlwimi 7,3,24,16,23          |         stwbrx 10,0,9
>         rlwimi 9,8,24,0,7            |         ld 3,-16(1)
>         rlwimi 9,8,24,16,23          <
>         sldi 7,7,32                  <
>         or 7,7,9                     <
>         mr 3,7                       <
>         blr                                    blr
>
> This same difference is responsible for the failure of pr58330.c which
> also uses __builtin_bswap64().
>
> The difference in RTL for the test case is this (after reload vs. after LRA):
> -   11: {%7:DI=bswap(%3:DI);clobber %8:DI;clobber %9:DI;clobber %10:DI;}
> -   20: %3:DI=%7:DI
> +   20: %8:DI=%1:DI-0x10
> +   21: %8:DI=%8:DI  // stupid no-op move
> +   11: {[%8:DI]=bswap(%3:DI);clobber %9:DI;clobber %10:DI;clobber scratch;}
> +   19: %3:DI=[%1:DI-0x10]
>
> So LRA believes going through memory is better than using a register,
> even though obviously there are plenty registers available.
>
> What LRA does:
>       Creating newreg=129
> Removing SCRATCH in insn #11 (nop 2)
>       Creating newreg=130
> Removing SCRATCH in insn #11 (nop 3)
>       Creating newreg=131
> Removing SCRATCH in insn #11 (nop 4)
> // at this point the insn would be a bswapdi2_64bit:
> //   11: {%3:DI=bswap(%3:DI);clobber r129;clobber r130;clobber r131;}
> // cost calculation for the insn alternatives:
>             0 Early clobber: reject++
>             1 Non-pseudo reload: reject+=2
>             1 Spill pseudo in memory: reject+=3
>             2 Scratch win: reject+=2
>             3 Scratch win: reject+=2
>             4 Scratch win: reject+=2
>           alt=0,overall=18,losers=1,rld_nregs=0
>             0 Non-pseudo reload: reject+=2
>             0 Spill pseudo in memory: reject+=3
>             0 Non input pseudo reload: reject++
>             2 Scratch win: reject+=2
>             3 Scratch win: reject+=2
>           alt=1,overall=16,losers=1,rld_nregs=0
>             Staticly defined alt reject+=12
>             0 Early clobber: reject++
>             2 Scratch win: reject+=2
>             3 Scratch win: reject+=2
>             4 Scratch win: reject+=2
>             0 Conflict early clobber reload: reject--
>           alt=2,overall=24,losers=1,rld_nregs=0
>          Choosing alt 1 in insn 11:  (0) Z  (1) r  (2) &b  (3) &r  (4)
> X {*bswapdi2_64bit}
>       Change to class BASE_REGS for r129
>       Change to class GENERAL_REGS for r130
>       Creating newreg=132 from oldreg=3, assigning class NO_REGS to r132
>       Change to class NO_REGS for r131
>    11: {r132:DI=bswap(%3:DI);clobber r129:DI;clobber r130:DI;clobber r131:DI;}
>       REG_UNUSED r131:DI
>       REG_UNUSED r130:DI
>       REG_UNUSED r129:DI
>
> LRA selects alternative 1 (Z,r,&b,&r,X) which seems to be the right
> choice, from looking at the constraints. Reload selects alternative 2
> which is slightly^2 discouraged: (??&r,r,&r,&r,&r).
>
> Is this an improvement or a regression? If it's an improvement then
> these two test cases should be adjusted :-)
>
> Ciao!
> Steven