[PATCH] simplify-rtx: Extend (truncate (*extract ...)) fold [PR87763]

[Richard Sandiford]

Maxim Kuvyrkov <maxim.kuvyrkov@gmail.com> writes:
> Hi Richard,
>
> You patch below increases code-size on aarch64-linux-gnu with -Os on SPEC2k6 400.perlbench and 453.povray -- by 1% and 2% respectively.
>
> 400.perlbench,perlbench_base.default,	101,939261,951221
> 453.povray,povray_base.default,		102,707807,721399
>
> Would you please check whether these can be avoided?  [Let me know if you need help reproducing this problem.]

I reverted the patch on Wednesday, see:
https://gcc.gnu.org/ml/gcc-patches/2020-01/msg01962.html

Thanks,
Richard

>
> Thank you,
>
> --
> Maxim Kuvyrkov
> https://www.linaro.org
>
>> On Jan 27, 2020, at 7:41 PM, Richard Sandiford <richard.sandiford@arm.com> wrote:
>> 
>> In the gcc.target/aarch64/lsl_asr_sbfiz.c part of this PR, we have:
>> 
>> Failed to match this instruction:
>> (set (reg:SI 95)
>>    (ashift:SI (subreg:SI (sign_extract:DI (subreg:DI (reg:SI 97) 0)
>>                (const_int 3 [0x3])
>>                (const_int 0 [0])) 0)
>>        (const_int 19 [0x13])))
>> 
>> If we perform the natural simplification to:
>> 
>> (set (reg:SI 95)
>>    (ashift:SI (sign_extract:SI (reg:SI 97)
>>                (const_int 3 [0x3])
>>                (const_int 0 [0])) 0)
>>        (const_int 19 [0x13])))
>> 
>> then the pattern matches.  And it turns out that we do have a
>> simplification like that already, but it would only kick in for
>> extractions from a reg, not a subreg.  E.g.:
>> 
>> (set (reg:SI 95)
>>    (ashift:SI (subreg:SI (sign_extract:DI (reg:DI X)
>>                (const_int 3 [0x3])
>>                (const_int 0 [0])) 0)
>>        (const_int 19 [0x13])))
>> 
>> would simplify to:
>> 
>> (set (reg:SI 95)
>>    (ashift:SI (sign_extract:SI (subreg:SI (reg:DI X) 0)
>>                (const_int 3 [0x3])
>>                (const_int 0 [0])) 0)
>>        (const_int 19 [0x13])))
>> 
>> IMO the subreg case is even more obviously a simplification
>> than the bare reg case, since the net effect is to remove
>> either one or two subregs, rather than simply change the
>> position of a subreg/truncation.
>> 
>> However, doing that regressed gcc.dg/tree-ssa/pr64910-2.c
>> for -m32 on x86_64-linux-gnu, because we could then simplify
>> a :HI zero_extract to a :QI one.  The associated *testqi_ext_3
>> pattern did already seem to want to handle QImode extractions:
>> 
>>  "ix86_match_ccmode (insn, CCNOmode)
>>   && ((TARGET_64BIT && GET_MODE (operands[2]) == DImode)
>>       || GET_MODE (operands[2]) == SImode
>>       || GET_MODE (operands[2]) == HImode
>>       || GET_MODE (operands[2]) == QImode)
>> 
>> but I'm not sure how often the QI case would trigger in practice,
>> since the zero_extract mode was restricted to HI and above.  I checked
>> the other x86 patterns and couldn't see any other instances of this.
>> 
>> Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linux-gnu,
>> OK to install?
>> 
>> Richard
>> 
>> 
>> 2020-01-27  Richard Sandiford  <richard.sandiford@arm.com>
>> 
>> gcc/
>> 	PR rtl-optimization/87763
>> 	* simplify-rtx.c (simplify_truncation): Extend sign/zero_extract
>> 	simplification to handle subregs as well as bare regs.
>> 	* config/i386/i386.md (*testqi_ext_3): Match QI extracts too.
>> ---
>> gcc/config/i386/i386.md | 2 +-
>> gcc/simplify-rtx.c      | 4 +++-
>> 2 files changed, 4 insertions(+), 2 deletions(-)
>> 
>> diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
>> index 6e9c9bd2fb6..a125ab350bb 100644
>> --- a/gcc/config/i386/i386.md
>> +++ b/gcc/config/i386/i386.md
>> @@ -8927,7 +8927,7 @@ (define_insn "*testqi_ext_2"
>> (define_insn_and_split "*testqi_ext_3"
>>   [(set (match_operand 0 "flags_reg_operand")
>>         (match_operator 1 "compare_operator"
>> -	  [(zero_extract:SWI248
>> +	  [(zero_extract:SWI
>> 	     (match_operand 2 "nonimmediate_operand" "rm")
>> 	     (match_operand 3 "const_int_operand" "n")
>> 	     (match_operand 4 "const_int_operand" "n"))
>> diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
>> index eff1d07a253..db4f9339c15 100644
>> --- a/gcc/simplify-rtx.c
>> +++ b/gcc/simplify-rtx.c
>> @@ -736,7 +736,9 @@ simplify_truncation (machine_mode mode, rtx op,
>>      (*_extract:M1 (truncate:M1 (reg:M2)) (len) (pos')) if possible without
>>      changing len.  */
>>   if ((GET_CODE (op) == ZERO_EXTRACT || GET_CODE (op) == SIGN_EXTRACT)
>> -      && REG_P (XEXP (op, 0))
>> +      && (REG_P (XEXP (op, 0))
>> +	  || (SUBREG_P (XEXP (op, 0))
>> +	      && REG_P (SUBREG_REG (XEXP (op, 0)))))
>>       && GET_MODE (XEXP (op, 0)) == GET_MODE (op)
>>       && CONST_INT_P (XEXP (op, 1))
>>       && CONST_INT_P (XEXP (op, 2)))
>> -- 
>> 2.17.1
>>