Re: [PATCH] combine: Narrow comparison of memory and constant

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/12/23 01:57, Stefan Schulze Frielinghaus via Gcc-patches wrote:
> Comparisons between memory and constants might be done in a smaller mode
> resulting in smaller constants which might finally end up as immediates
> instead of in the literal pool.
> 
> For example, on s390x a non-symmetric comparison like
>    x <= 0x3fffffffffffffff
> results in the constant being spilled to the literal pool and an 8 byte
> memory comparison is emitted.  Ideally, an equivalent comparison
>    x0 <= 0x3f
> where x0 is the most significant byte of x, is emitted where the
> constant is smaller and more likely to materialize as an immediate.
> 
> Similarly, comparisons of the form
>    x >= 0x4000000000000000
> can be shortened into x0 >= 0x40.
> 
> I'm not entirely sure whether combine is the right place to implement
> something like this.  In my first try I implemented it in
> TARGET_CANONICALIZE_COMPARISON but then thought other targets might
> profit from it, too.  simplify_context::simplify_relational_operation_1
> seems to be the wrong place since code/mode may change.  Any opinions?
> 
> gcc/ChangeLog:
> 
> 	* combine.cc (simplify_compare_const): Narrow comparison of
> 	memory and constant.
> 	(try_combine): Adapt new function signature.
> 	(simplify_comparison): Adapt new function signature.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.target/s390/cmp-mem-const-1.c: New test.
> 	* gcc.target/s390/cmp-mem-const-2.c: New test.
This does seem more general than we'd want to do in the canonicalization 
hook.  So thanks for going the extra mile and doing a generic 
implementation.




> @@ -11987,6 +11988,79 @@ simplify_compare_const (enum rtx_code code, machine_mode mode,
>         break;
>       }
>   
> +  /* Narrow non-symmetric comparison of memory and constant as e.g.
> +     x0...x7 <= 0x3fffffffffffffff into x0 <= 0x3f where x0 is the most
> +     significant byte.  Likewise, transform x0...x7 >= 0x4000000000000000 into
> +     x0 >= 0x40.  */
> +  if ((code == LEU || code == LTU || code == GEU || code == GTU)
> +      && is_a <scalar_int_mode> (GET_MODE (op0), &int_mode)
> +      && MEM_P (op0)
> +      && !MEM_VOLATILE_P (op0)
> +      && (unsigned HOST_WIDE_INT)const_op > 0xff)
> +    {
> +      unsigned HOST_WIDE_INT n = (unsigned HOST_WIDE_INT)const_op;
> +      enum rtx_code adjusted_code = code;
> +
> +      /* If the least significant bit is already zero, then adjust the
> +	 comparison in the hope that we hit cases like
> +	   op0  <= 0x3ffffdfffffffffe
> +	 where the adjusted comparison
> +	   op0  <  0x3ffffdffffffffff
> +	 can be shortened into
> +	   op0' <  0x3ffffd.  */
> +      if (code == LEU && (n & 1) == 0)
> +	{
> +	  ++n;
> +	  adjusted_code = LTU;
> +	}
> +      /* or e.g. op0 < 0x4020000000000000  */
> +      else if (code == LTU && (n & 1) == 0)
> +	{
> +	  --n;
> +	  adjusted_code = LEU;
> +	}
> +      /* or op0 >= 0x4000000000000001  */
> +      else if (code == GEU && (n & 1) == 1)
> +	{
> +	  --n;
> +	  adjusted_code = GTU;
> +	}
> +      /* or op0 > 0x3fffffffffffffff.  */
> +      else if (code == GTU && (n & 1) == 1)
> +	{
> +	  ++n;
> +	  adjusted_code = GEU;
> +	}
> +
> +      scalar_int_mode narrow_mode_iter;
> +      bool lower_p = code == LEU || code == LTU;
> +      bool greater_p = !lower_p;
> +      FOR_EACH_MODE_UNTIL (narrow_mode_iter, int_mode)
> +	{
> +	  unsigned nbits = GET_MODE_PRECISION (int_mode)
> +			  - GET_MODE_PRECISION (narrow_mode_iter);
> +	  unsigned HOST_WIDE_INT mask = (HOST_WIDE_INT_1U << nbits) - 1;
> +	  unsigned HOST_WIDE_INT lower_bits = n & mask;
> +	  if ((lower_p && lower_bits == mask)
> +	      || (greater_p && lower_bits == 0))
> +	    {
> +	      n >>= nbits;
> +	      break;
> +	    }
> +	}
> +
> +      if (narrow_mode_iter < int_mode)
> +	{
> +	  poly_int64 offset = BYTES_BIG_ENDIAN
> +				? 0
> +				: GET_MODE_SIZE (int_mode)
> +				  - GET_MODE_SIZE (narrow_mode_iter);
Go ahead and add some parenthesis here.  I'd add one pair around the 
whole RHS of that assignment.  The '?' and ':' would line up under the 
'B' in that case.  Similarly add them around the false arm of the 
ternary.  The '-' will line up under the 'G'.

Going to trust you got the little endian adjustment correct here ;-)


>   
>         /* Compute some predicates to simplify code below.  */
> diff --git a/gcc/testsuite/gcc.target/s390/cmp-mem-const-1.c b/gcc/testsuite/gcc.target/s390/cmp-mem-const-1.c
> new file mode 100644
> index 00000000000..b90c2a8c224
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/s390/cmp-mem-const-1.c
> @@ -0,0 +1,99 @@
> +/* { dg-do compile { target { lp64 } } } */
> +/* { dg-options "-O1 -march=z13 -mzarch" } */
> +/* { dg-final { scan-assembler-not {\tclc\t} } } */
> +
> +int
> +ge_8b (unsigned long *x)
> +{
> +  return *x >= 0x4000000000000000;
> +}
Would it be possible to add some debugging output in 
simplify_compare_const so that you could search for that debugging 
output and make these tests generic?

Jeff