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

On 06/08/14 23:29, Richard Biener wrote:
> On Wed, Aug 6, 2014 at 3:21 PM, Kugan <> wrote:
>> On 06/08/14 22:09, Richard Biener wrote:
>>> On Tue, Aug 5, 2014 at 4:21 PM, Jakub Jelinek <> wrote:
>>>> On Tue, Aug 05, 2014 at 04:17:41PM +0200, Richard Biener wrote:
>>>>> what's the semantic of setting SRP_SIGNED_AND_UNSIGNED
>>>>> on the subreg?  That is, for the created (subreg:lhs_mode
>>>>> (reg:<PROMOTE_MODE of ssa> N))?
>>>> SRP_SIGNED_AND_UNSIGNED on a subreg should mean that
>>>> the subreg is both zero and sign extended, which means
>>>> that the topmost bit of the narrower mode is known to be zero,
>>>> and all bits above it in the wider mode are known to be zero too.
>>>> SRP_SIGNED means that the topmost bit of the narrower mode is
>>>> either 0 or 1 and depending on that the above wider mode bits
>>>> are either all 0 or all 1.
>>>> SRP_UNSIGNED means that regardless of the topmost bit value,
>>>> all above wider mode bits are 0.
>>> Ok, then from the context of the patch we already know that
>>> either SRP_UNSIGNED or SRP_SIGNED is true which means
>>> that the value is sign- or zero-extended.
>>> I suppose inside promoted_for_type_p
>>> TYPE_MODE (TREE_TYPE (ssa)) == lhs_mode, I'm not sure
>>> why you pass !unsignedp as lhs_uns.
>> In expand_expr_real_1, it is already known that it is promoted for
>> unsigned_p and we are setting SUBREG_PROMOTED_SET (temp, unsignedp).
>> If we can prove that it is also promoted for !unsignedp, we can set
>> promoted_for_type_p should prove this based on the value range info.
>>> Now, from 'ssa' alone we can't tell anything about a larger mode
>>> registers value if that is either zero- or sign-extended.  But we
>>> know that those bits are properly zero-extended if unsignedp
>>> and properly sign-extended if !unsignedp?
>>> So what the predicate tries to prove is that sign- and zero-extending
>>> results in the same larger-mode value.  This is true if the
>>> MSB of the smaller mode is not set.
>>> Let's assume that smaller mode is that of 'ssa' then the test
>>> is just
>>>   return (!tree_int_cst_sign_bit (min) && !tree_int_cst_sign_bit (max));
>>> no?
>> hmm,  is this because we will never have a call to promoted_for_type_p
>> with same sign (ignoring PROMOTE_MODE) for 'ssa' and the larger mode.
>> The case with larger mode signed and 'ssa' unsigned will not work.
>> Therefore larger mode unsigned and 'ssa' signed will be the only case
>> that we should consider.
>> However, with PROMOTE_MODE, isnt that we will miss some cases with this.
> No, PROMOTE_MODE will still either sign- or zero-extend.  If either
> results in zeros in the upper bits then PROMOTE_MODE doesn't matter.

Thanks for the explanation. Please find the attached patch that
implements this. I have updated the comments and predicate to match this.

Bootstrap tested on x86_64-unknown-linux-gnu and regression tested on
x86_64-unknown-linux-gnu and arm-none-linux-gnueabi with no new
regressions. Is this OK?


2014-08-07  Kugan Vivekanandarajah  <>

	* 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

2014-08-07  Kugan Vivekanandarajah  <>

	* gcc.dg/zero_sign_ext_test.c: New test.

Attachment: p2.txt
Description: Text document

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