Re: combine_conversions int->double->int

[Richard Guenther <richard dot guenther at gmail dot com>]

On Wed, Apr 25, 2012 at 3:58 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Wed, 25 Apr 2012, Richard Guenther wrote:
>
>> On Wed, Apr 25, 2012 at 10:12 AM, Marc Glisse <marc.glisse@inria.fr>
>> wrote:
>>>
>>> Hello,
>>>
>>> a conversion like int->double->int is just the identity, as long as
>>> double
>>> is big enough to represent all ints exactly. The most natural way I found
>>> to
>>> do this optimization is the attached:
>>>
>>> 2012-04-25 ?Marc Glisse ?<marc.glisse@inria.fr>
>>>
>>> ? ? ? ?PR middle-end/27139
>>> ? ? ? ?* tree-ssa-forwprop.c (combine_conversions): Handle INT->FP->INT.
>>>
>>> Does the approach make sense? I don't know that code, and adding
>>> FLOAT_EXPR
>>> / FIX_TRUNC_EXPR was a bit of guesswork. The precision of double could be
>>> multiplied by log2(base), but not doing it is safe. If the approach is
>>> ok, I
>>> could extend it so int->double->long also skips the intermediate
>>> conversion.
>>>
>>> Bootstrapped and regression tested on linux-x86_64.
>>>
>>> Should I try and write a testcase for a specific target checking for
>>> specific asm instructions there, or is there a better way?
>>
>>
>> Well, scanning the forwprop dump for example.
>>
>> Btw, I think not munging this new case into the existing CONVERT_EXPR_P
>> code would be better - it makes the code (even) harder to understand and
>> I'm not convinced that adding FLOAT_EXPR/FIX_TRUNC_EXPR handling
>> does not wreck any assumptions in that code.
>>
>> It also seems that for DECIMAL_FLOAT_TYPE_P the transform is always
>> valid?
>>
>> Richard.
>>
>>> (note that I can't commit)
>
>
> Here is take 2 on this patch, which seems cleaner. Bootstrapped and
> regression tested.
>
> gcc/ChangeLog
>
> 2012-04-25 ?Marc Glisse ?<marc.glisse@inria.fr>
>
> ? ? ? ?PR middle-end/27139
> ? ? ? ?* tree-ssa-forwprop.c (combine_conversions): Handle INT->FP->INT.
>
> gcc/testsuite/ChangeLog
>
> 2012-04-25 ?Marc Glisse ?<marc.glisse@inria.fr>
>
> ? ? ? ?PR middle-end/27139
> ? ? ? ?* gcc.dg/tree-ssa/forwprop-18.c: New test.
>
>
> In my patch, the lines with gimple_assign_* are vaguely guessed from what is
> around, I don't pretend to understand them.

;)

The patch looks good to me - on a 2nd thought folding the case back in
to the if (CONVERT_EXPR_CODE_P (gimple_assign_rhs_code (def_stmt)))
block to benefit from the local vars therein makes sense (but as separate
sub-if () as it is now).

Thanks,
Richard.

>
> While doing this, I noticed what I think is a mistake in a comment:
>
> --- gcc/real.c ? ? ?(revision 186761)
> +++ gcc/real.c ? ? ?(working copy)
> @@ -2814,11 +2814,11 @@ significand_size (enum machine_mode mode
> ? ? return 0;
>
> ? if (fmt->b == 10)
> ? ? {
> ? ? ? /* Return the size in bits of the largest binary value that can be
> - ? ? ? ?held by the decimal coefficient for this mode. ?This is one more
> + ? ? ? ?held by the decimal coefficient for this mode. ?This is one less
> ? ? ? ? than the number of bits required to hold the largest coefficient
> ? ? ? ? of this mode. ?*/
> ? ? ? double log2_10 = 3.3219281;
> ? ? ? return fmt->p * log2_10;
> ? ? }
>
> --
> Marc Glisse