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Re: [PING^2][PATCH][2 of 2] RTL expansion for zero sign extension elimination with VRP


On Tue, 15 Oct 2013, Kugan wrote:

> Hi Eric,
> 
> Can you please help to review this patch?
> http://gcc.gnu.org/ml/gcc-patches/2013-10/msg00452.html

I think that gimple_assign_is_zero_sign_ext_redundant and its
description is somewhat confused.  You seem to have two cases
here, one being NOP_EXPR or CONVERT_EXPR that are in the IL
because they are required for type correctness.  So,

   long = (long) int

which usually expands to

   (set:DI (sext:DI reg:SI))

you want to expand as

   (set:DI (subreg:DI reg:SI))

where you have to be careful for modes smaller than word_mode.
You don't seem to implement this optimization though (but
the gimple_assign_is_zero_sign_ext_redundant talks about it).

Second are promotions required by the target (PROMOTE_MODE)
that do arithmetic on wider registers like for

  char = char + char

where they cannot do

  (set:QI (plus:QI reg:QI reg:QI))

but instead have, for example reg:SI only and you get

  (set:SI (plus:SI reg:SI reg:SI))
  (set:SI ([sz]ext:SI (subreg:QI reg:SI)))

that you try to address with the cfgexpand hunk but I believe
it doesn't work the way you do it.  That is because on GIMPLE
you do not see SImode temporaries and thus no SImode value-ranges.
Consider

  tem = (char) 255 + (char) 1;

which has a value-range of [0,0] but clearly when computed in
SImode the value-range is [256, 256].  That is, VRP computes
value-ranges in the expression type, not in some arbitrary
larger type.

So what you'd have to do is take the value-ranges of the
two operands of the plus and see whether the plus can overflow
QImode when computed in SImode (for the example).

[exposing the effect of PROMOTE_MODE earlier than at RTL expansion
time may make this less awkward]

Thanks,
Richard.

> Thanks,
> Kugan
> 
> > +2013-09-25  Kugan Vivekanandarajah  <kuganv@linaro.org>
> > +
> > +	* dojump.c (do_compare_and_jump): Generate rtl without
> > +	zero/sign extension if redundant.
> > +	* cfgexpand.c (expand_gimple_stmt_1): Likewise.
> > +	* gimple.c (gimple_assign_is_zero_sign_ext_redundant) : New
> > +	function.
> > +	* gimple.h (gimple_assign_is_zero_sign_ext_redundant) : Declare.
> > +
> > 
> > 


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