* Postreload and STRICT_LOW_PART
@ 2009-09-20 16:10 Andreas Schwab
2009-09-20 16:31 ` Richard Guenther
0 siblings, 1 reply; 3+ messages in thread
From: Andreas Schwab @ 2009-09-20 16:10 UTC (permalink / raw)
To: gcc
Why is postreload converting (set (REGX) (CONST_INT A)) ... (set (REGX)
(CONST_INT B)) into (set (STRICT_LOW_PART (REGX)) (CONST_INT B))? That
looks like a pessimisation especially if the constants are small, since
STRICT_LOW_PART must not touch the high part. Is there a way for the
backend to stop postreload from doing this if the constants are in some
range? On the m68k, loading a constant in the range -128..127 in SI
mode is better than loading it in strict QI mode.
Andreas.
--
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5
"And now for something completely different."
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Postreload and STRICT_LOW_PART
2009-09-20 16:10 Postreload and STRICT_LOW_PART Andreas Schwab
@ 2009-09-20 16:31 ` Richard Guenther
2009-09-20 17:25 ` Paolo Bonzini
0 siblings, 1 reply; 3+ messages in thread
From: Richard Guenther @ 2009-09-20 16:31 UTC (permalink / raw)
To: Andreas Schwab; +Cc: gcc
On Sun, Sep 20, 2009 at 6:10 PM, Andreas Schwab <schwab@linux-m68k.org> wrote:
> Why is postreload converting (set (REGX) (CONST_INT A)) ... (set (REGX)
> (CONST_INT B)) into (set (STRICT_LOW_PART (REGX)) (CONST_INT B))? That
> looks like a pessimisation especially if the constants are small, since
> STRICT_LOW_PART must not touch the high part. Is there a way for the
> backend to stop postreload from doing this if the constants are in some
> range? On the m68k, loading a constant in the range -128..127 in SI
> mode is better than loading it in strict QI mode.
It's probably an omission of a check if A & ~GET_MODE_MASK (narrow_mode)
is equal to zero. And of course a cost check is completely missing.
Richard.
> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5
> "And now for something completely different."
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Postreload and STRICT_LOW_PART
2009-09-20 16:31 ` Richard Guenther
@ 2009-09-20 17:25 ` Paolo Bonzini
0 siblings, 0 replies; 3+ messages in thread
From: Paolo Bonzini @ 2009-09-20 17:25 UTC (permalink / raw)
To: Richard Guenther; +Cc: Andreas Schwab, gcc
On 09/20/2009 06:31 PM, Richard Guenther wrote:
> On Sun, Sep 20, 2009 at 6:10 PM, Andreas Schwab<schwab@linux-m68k.org> wrote:
>> Why is postreload converting (set (REGX) (CONST_INT A)) ... (set (REGX)
>> (CONST_INT B)) into (set (STRICT_LOW_PART (REGX)) (CONST_INT B))? That
>> looks like a pessimisation especially if the constants are small, since
>> STRICT_LOW_PART must not touch the high part. Is there a way for the
>> backend to stop postreload from doing this if the constants are in some
>> range? On the m68k, loading a constant in the range -128..127 in SI
>> mode is better than loading it in strict QI mode.
>
> It's probably an omission of a check if A& ~GET_MODE_MASK (narrow_mode)
> is equal to zero.
Actually it is (A^B) & ~GET_MODE_MASK(narrow_mode) that has to be 0.
> And of course a cost check is completely missing.
I think that adding a cost check would be the right thing to do. For
size, a QImode move is probably better (it is on i386 for example).
Paolo
^ permalink raw reply [flat|nested] 3+ messages in thread
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2009-09-20 16:10 Postreload and STRICT_LOW_PART Andreas Schwab
2009-09-20 16:31 ` Richard Guenther
2009-09-20 17:25 ` Paolo Bonzini
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