Re: [PING][PATCH] RISC-V: Avoid redundant sign-extension for SImode SGE, SGEU, SLE, SLEU

[Jeff Law <jeffreyalaw@gmail.com>]

On 11/28/22 08:38, Maciej W. Rozycki wrote:
> On Mon, 28 Nov 2022, Jeff Law wrote:
>
>>>>> LGTM, but with a nit, I don't get set.w but get an andi like below, so
>>>>> maybe we should also scan-assembler-not andi? feel free to commit that
>>>>> directly with that fix
>>>>>
>>>>> ```asm
>>>>> sleu:
>>>>>          sgtu    a0,a0,a1        # 9     [c=4 l=4]  *sgtu_disi
>>>>>          xori    a0,a0,1 # 10    [c=4 l=4]  *xorsi3_internal/1
>>>>>          andi    a0,a0,1 # 16    [c=4 l=4]  anddi3/1
>>>>>          ret             # 25    [c=0 l=4]  simple_return
>>>>> ```
>>>>    Interesting.  I can do that, but can you please share the compilation
>>>> options, given or defaulted (from `--with...' configuration options), this
>>>> happens with?
>>>    I have noticed it went nowhere.  Can you please check what compilation
>>> options lead to this discrepancy so that we can have the fix included in
>>> GCC 13?  I'd like to understand what's going on here.
>> FWIW, I don't see the redundant sign extension with this testcase at -O2 on
>> the trunk.  Is it possible the patch has been made redundant over the last few
>> months?
>   Maybe at -O2, but the test cases continue to fail in my configuration for
> other optimisation levels:
>
> FAIL: gcc.target/riscv/sge.c   -O1   scan-assembler-not sext\\.w
> FAIL: gcc.target/riscv/sge.c  -Og -g   scan-assembler-not sext\\.w
> FAIL: gcc.target/riscv/sgeu.c   -O1   scan-assembler-not sext\\.w
> FAIL: gcc.target/riscv/sgeu.c  -Og -g   scan-assembler-not sext\\.w
> FAIL: gcc.target/riscv/sle.c   -O1   scan-assembler-not sext\\.w
> FAIL: gcc.target/riscv/sle.c  -Og -g   scan-assembler-not sext\\.w
> FAIL: gcc.target/riscv/sleu.c   -O1   scan-assembler-not sext\\.w
> FAIL: gcc.target/riscv/sleu.c  -Og -g   scan-assembler-not sext\\.w

I may have been running an rv32 toolchain...  So I'll start over and 
ensure that I'm running rv64 :-)


With the trunk, I get code like Kito (AND with 0x1 mask)


The key difference is Roger's patch:

commit c23a9c87cc62bd177fd0d4db6ad34b34e1b9a31f
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed Aug 3 08:55:35 2022 +0100

     Some additional zero-extension related optimizations in simplify-rtx.

     This patch implements some additional zero-extension and sign-extension
     related optimizations in simplify-rtx.cc.  The original motivation 
comes
     from PR rtl-optimization/71775, where in comment #2 Andrew Pinksi sees:

     Failed to match this instruction:
     (set (reg:DI 88 [ _1 ])
         (sign_extend:DI (subreg:SI (ctz:DI (reg/v:DI 86 [ x ])) 0)))

[ ... ]

With that patch the sign extension is removed and instead we generate 
the AND with 0x1.

Old, from combine dump:

   Successfully matched this instruction:
   (set (reg/i:DI 10 a0)
!     (sign_extend:DI (reg:SI 78)))


New, from combine dump:

   (set (reg/i:DI 10 a0)
!     (and:DI (subreg:DI (reg:SI 78) 0)
!         (const_int 1 [0x1])))

Note the date on Roger's patch, roughly the same time as yours. I 
suspect Kito had tested the truck with Roger's patch.


Your patch is probably still useful.  I think Kito's only concern was to 
make sure we don't have the ANDI instruction in addition to not having 
the SEXT instruction.  So still approved for trunk, just update the 
testcases to make sure we don't have the ANDI too.


jeff