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

[Kugan <kugan.vivekanandarajah@linaro.org>]

Thanks foe the review and suggestions.

On 10/07/14 22:15, Richard Biener wrote:
> On Mon, Jul 7, 2014 at 8:55 AM, Kugan <kugan.vivekanandarajah@linaro.org> wrote:

[...]

>>
>> For -fwrapv, it is due to how PROMOTE_MODE is defined in arm back-end.
>> In the test-case, a function (which has signed char return type) returns
>> -1 in one of the paths. ARM PROMOTE_MODE changes that to 255 and relies
>> on zero/sign extension generated by RTL again for the correct value. I
>> saw some other targets also defining similar think. I am therefore
>> skipping removing zero/sign extension if the ssa variable can be set to
>> negative integer constants.
> 
> Hm?  I think you should rather check that you are removing a
> sign-/zero-extension - PROMOTE_MODE tells you if it will sign- or
> zero-extend.  Definitely
> 
> +  /* In some architectures, negative integer constants are truncated and
> +     sign changed with target defined PROMOTE_MODE macro. This will impact
> +     the value range seen here and produce wrong code if zero/sign extensions
> +     are eliminated. Therefore, return false if this SSA can have negative
> +     integers.  */
> +  if (is_gimple_assign (stmt)
> +      && (TREE_CODE_CLASS (gimple_assign_rhs_code (stmt)) == tcc_unary))
> +    {
> +      tree rhs1 = gimple_assign_rhs1 (stmt);
> +      if (TREE_CODE (rhs1) == INTEGER_CST
> +         && !TYPE_UNSIGNED (TREE_TYPE (ssa))
> +         && tree_int_cst_compare (rhs1, integer_zero_node) == -1)
> +       return false;
> 
> looks completely bogus ... (an unary op with a constant operand?)
> instead you want to do sth like

I see that unary op with a constant operand is not possible in gimple.
What I wanted to check here is any sort of constant loads; but seems
that will not happen in gimple. Is PHI statements the only possible
statements where we will end up with such constants.

>   mode = TYPE_MODE (TREE_TYPE (ssa));
>   rhs_uns = TYPE_UNSIGNED (TREE_TYPE (ssa));
>   PROMOTE_MODE (mode, rhs_uns, TREE_TYPE (ssa));
> 
> instead of initializing rhs_uns from ssas type.  That is, if
> PROMOTE_MODE tells you to promote _not_ according to ssas sign then
> honor that.

This is triggered in pr43017.c in function foo for arm-none-linux-gnueabi.

where, the gimple statement that cause this looks like:
.....
  # _3 = PHI <_17(7), -1(2)>
bb43:
  return _3;

ARM PROMOTE_MODE changes the sign for integer constants only and hence
looking at the variable with PROMOTE_MODE is not changing the sign in
this case.

#define PROMOTE_MODE(MODE, UNSIGNEDP, TYPE)	\
  if (GET_MODE_CLASS (MODE) == MODE_INT		\
      && GET_MODE_SIZE (MODE) < 4)      	\
    {						\
      if (MODE == QImode)			\
	UNSIGNEDP = 1;				\
      else if (MODE == HImode)			\
	UNSIGNEDP = 1;				\
      (MODE) = SImode;				\
    }

>> As for the -fno-strict-overflow case, if the variables overflows, in VRP
>> dumps, I see +INF(OVF), but the value range stored in ssa has TYPE_MAX.
>> We therefore should limit the comparison to (TYPE_MIN < VR_MIN && VR_MAX
>> < TYPE_MAX) instead of (TYPE_MIN <= VR_MIN && VR_MAX <= TYPE_MAX) when
>> checking to be sure that this is not the overflowing case. Attached
>> patch changes this.
> 
> I don't think that's necessary - the overflow cases happen only when
> that overflow has undefined behavior, thus any valid program will have
> values <= MAX.

I see that you have now removed +INF(OVF). I will change it this way.

Thanks again,
Kugan