Re: [PATCH, v2] rs6000: Vector shift-right should honor modulo semantics

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi Bill,

On Mon, Feb 11, 2019 at 07:36:11AM -0600, Bill Schmidt wrote:
> 2019-02-11  Bill Schmidt  <wschmidt@linux.ibm.com>
> 
> 	* config/rs6000/rs6000.c (rs6000_gimple_fold_builtin): Shift-right
> 	and shift-left vector built-ins need to include a TRUNC_MOD_EXPR
> 	for correct semantics.
> 
> [gcc/testsuite]
> 
> 2019-02-11  Bill Schmidt  <wschmidt@linux.ibm.com>
> 
> 	* gcc.target/powerpc/vec-sld-modulo.c: New.
> 	* gcc.target/powerpc/vec-srad-modulo.c: New.
> 	* gcc.target/powerpc/vec-srd-modulo.c: New.

This is okay for trunk and backports.  Thanks!

One comment:

> +vec_sldi (vui64_t vra, const unsigned int shb)
> +{
> +  vui64_t lshift;
> +  vui64_t result;
> +
> +  /* Note legitimate use of wrong-type splat due to expectation that only
> +     lower 6-bits are read.  */
> +  lshift = (vui64_t) vec_splat_s8((const unsigned char)shb);
> +
> +  /* Vector Shift Left Doublewords based on the lower 6-bits
> +     of corresponding element of lshift.  */
> +  result = vec_vsld (vra, lshift);
> +
> +  return (vui64_t) result;
> +}

I realise this is a testcase, and in one frame of mind it is good to test
all different styles and bad habits.  But please never use casts that do not
do anything in correct programs: the only thing such casts do is they shut
up warnings in incorrect programs (including the same program after a wrong
change).  </petpeeve>


Segher