Re: [PATCH 2/2] Enable elimination of zext/sext

[Kugan <kugan dot vivekanandarajah at linaro dot org>]

On 08/09/14 19:48, Richard Biener wrote:
> On Sun, Sep 7, 2014 at 11:50 AM, Kugan
> <kugan.vivekanandarajah@linaro.org> wrote:
>> On 05/09/14 19:50, Richard Biener wrote:
>>
>>> Well - the best way would be to expose the target specifics to GIMPLE
>>> at some point in the optimization pipeline.  My guess would be that it's
>>> appropriate after loop optimizations (but maybe before induction variable
>>> optimization).
>>>
>>> That is, have a pass that applies register promotion to all SSA names
>>> in the function, inserting appropriate truncations and extensions.  That
>>> way you'd never see (set (subreg...) on RTL.  The VRP and DOM
>>> passes running after that pass would then be able to aggressively
>>> optimize redundant truncations and extensions.
>>>
>>> Effects on debug information are to be considered.  You can change
>>> the type of SSA names in-place but you don't want to do that for
>>> user DECLs (and we can't have the SSA name type and its DECL
>>> type differ - and not sure if we might want to lift that restriction).
>>
>> Thanks. I will try to implement this.
>>
>> I still would like to keep the VRP based approach as there are some
>> cases that I think can only be done with range info. For example:
>>
>> short foo(unsigned char c)
>> {
>>   c = c & (unsigned char)0x0F;
>>   if( c > 7 )
>>     return((short)(c - 5));
>>   else
>>     return(( short )c);
>> }
>>
>>
>> So, how about adding and setting the overflow/wrap around flag to
>> range_info. We now set static_flag for VR_RANG/VR_ANTI_RANGE. If we go
>> back to the max + 1, min - 1 for VR_ANTI_RANGE, we can use this
>> static_flag to encode overflow/wrap around. Will that be something
>> acceptable?
> 
> You mean tracking in the VRP lattice whether a value wrapped around
> (or was assumed not to due to undefined behavior)?  I'm not sure this
> is easy to do correctly (VRP is large).
> 
> Note that I don't think we'll lose the testcase you quoted if the promotion
> pass runs before VRP2.   We'd have as input to VRP2 sth like (assuming
> promote mode would promote to SImode)
> 
>   SImode tem_2 = (unsigned int)c_1(D);
>   tem_3 = tem_3 & 0xF;
>   if (tem_3 > 7)
>     {
>       tem_4 = tem_3 - 5;
>       short _5 = (short)_4;
>       tem_5 = (unsigned int)_5;
>      return tem_5;
>    }
> else
>    {
>      short _6 = (short)_3;
>      return _6;
>    }
> 
> VRP should be able to remove the (unsigned int)(short) sign-extension
> of tem_4.
> 
> note that both incoming registers and return registers are "interesting".
> For simplicity I suggest to not promote them on GIMPLE.
> 
> What you'd lose in VRP2 is the smaller value-ranges you'd get from
> (undefined) wrapping.  You could recover the undefinedness by
> looking at SSA names recorded value-range and transfering that
> in the promotion pass (but I'm not sure if you want to open the
> can of latent signed overflow bugs in programs even more for
> PROMOTE_MODE targets...)
> 

Thanks. In the meantime I would like to revert the patch which is
enabling zero/sign extension. I have bootstrapped it in x86_64 and
regression testing is ongoing. Is this OK ?

Thanks,
Kugan

gcc/ChangeLog:

2014-09-09  Kugan Vivekanandarajah  <kuganv@linaro.org>

	Revert r213751:
	* calls.c (precompute_arguments): Check
	 promoted_for_signed_and_unsigned_p and set the promoted mode.
	(promoted_for_signed_and_unsigned_p): New function.
	(expand_expr_real_1): Check promoted_for_signed_and_unsigned_p
	and set the promoted mode.
	* expr.h (promoted_for_signed_and_unsigned_p): New function definition.
	* cfgexpand.c (expand_gimple_stmt_1): Call emit_move_insn if
	SUBREG is promoted with SRP_SIGNED_AND_UNSIGNED.

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