Re: [PATCH] Optimized "(X - N * M) / N + M" to "X / N" if valid

[Richard Biener <rguenther@suse.de>]

On Wed, 17 May 2023, Jiufu Guo wrote:

> Hi,
> 
> This patch tries to optimize "(X - N * M) / N + M" to "X / N".

But if that's valid why not make the transform simpler and transform
(X - N * M) / N  to X / N - M instead?

You use the same optimize_x_minus_NM_div_N_plus_M validator for
the division and shift variants but the overflow rules are different,
so I'm not sure that's warranted.  I'd also prefer to not split out
the validator to a different file - iff then the appropriate file
is fold-const.cc, not gimple-match-head.cc (I see we're a bit
inconsistent here, for pure gimple matches gimple-fold.cc would
be another place).

Since you use range information why is the transform restricted
to constant M?

Richard.

> As per the discussions in PR108757, we know this transformation is valid
> only under some conditions.
> For C code, "/" towards zero (trunc_div), and "X - N * M"
> maybe wrap/overflow/underflow. So, it is valid that "X - N * M" does
> not cross zero and does not wrap/overflow/underflow.
> 
> This patch also handles the case when "N" is the power of 2, where
> "(X - N * M) / N" is "(X - N * M) >> log2(N)".
> 
> Bootstrap & regtest pass on ppc64{,le} and x86_64.
> Is this ok for trunk?
> 
> BR,
> Jeff (Jiufu)
> 
> 	PR tree-optimization/108757
> 
> gcc/ChangeLog:
> 
> 	* gimple-match-head.cc (optimize_x_minus_NM_div_N_plus_M): New function.
> 	* match.pd ((X - N * M) / N + M): New pattern.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.dg/pr108757-1.c: New test.
> 	* gcc.dg/pr108757-2.c: New test.
> 	* gcc.dg/pr108757.h: New test.
> 
> ---
>  gcc/gimple-match-head.cc          |  54 ++++++++++
>  gcc/match.pd                      |  22 ++++
>  gcc/testsuite/gcc.dg/pr108757-1.c |  17 ++++
>  gcc/testsuite/gcc.dg/pr108757-2.c |  18 ++++
>  gcc/testsuite/gcc.dg/pr108757.h   | 160 ++++++++++++++++++++++++++++++
>  5 files changed, 271 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/pr108757-1.c
>  create mode 100644 gcc/testsuite/gcc.dg/pr108757-2.c
>  create mode 100644 gcc/testsuite/gcc.dg/pr108757.h
> 
> diff --git a/gcc/gimple-match-head.cc b/gcc/gimple-match-head.cc
> index b08cd891a13..680a4cb2fc6 100644
> --- a/gcc/gimple-match-head.cc
> +++ b/gcc/gimple-match-head.cc
> @@ -224,3 +224,57 @@ optimize_successive_divisions_p (tree divisor, tree inner_div)
>      }
>    return true;
>  }
> +
> +/* Return true if "(X - N * M) / N + M" can be optimized into "X / N".
> +   Otherwise return false.
> +
> +   For unsigned,
> +   If sign bit of M is 0 (clz is 0), valid range is [N*M, MAX].
> +   If sign bit of M is 1, valid range is [0, MAX - N*(-M)].
> +
> +   For signed,
> +   If N*M > 0, valid range: [MIN+N*M, 0] + [N*M, MAX]
> +   If N*M < 0, valid range: [MIN, -(-N*M)] + [0, MAX - (-N*M)].  */
> +
> +static bool
> +optimize_x_minus_NM_div_N_plus_M (tree x, wide_int n, wide_int m, tree type)
> +{
> +  wide_int max = wi::max_value (type);
> +  signop sgn = TYPE_SIGN (type);
> +  wide_int nm;
> +  wi::overflow_type ovf;
> +  if (TYPE_UNSIGNED (type) && wi::clz (m) == 0)
> +    nm = wi::mul (n, -m, sgn, &ovf);
> +  else
> +    nm = wi::mul (n, m, sgn, &ovf);
> +
> +  if (ovf != wi::OVF_NONE)
> +    return false;
> +
> +  value_range vr0;
> +  if (!get_range_query (cfun)->range_of_expr (vr0, x) || vr0.varying_p ()
> +      || vr0.undefined_p ())
> +    return false;
> +
> +  wide_int wmin0 = vr0.lower_bound ();
> +  wide_int wmax0 = vr0.upper_bound ();
> +  wide_int min = wi::min_value (type);
> +
> +  /* unsigned */
> +  if ((TYPE_UNSIGNED (type)))
> +    /* M > 0 (clz != 0): [N*M, MAX],  M < 0 : [0, MAX-N*(-M)]  */
> +    return wi::clz (m) != 0 ? wi::ge_p (wmin0, nm, sgn)
> +			    : wi::le_p (wmax0, max - nm, sgn);
> +
> +  /* signed, N*M > 0 */
> +  else if (wi::gt_p (nm, 0, sgn))
> +    /* [N*M, MAX] or [MIN+N*M, 0] */
> +    return wi::ge_p (wmin0, nm, sgn)
> +	   || (wi::ge_p (wmin0, min + nm, sgn) && wi::le_p (wmax0, 0, sgn));
> +
> +  /* signed, N*M < 0 */
> +  /* [MIN, N*M] or [0, MAX + N*M]*/
> +  else
> +    return wi::le_p (wmax0, nm, sgn)
> +	   || (wi::ge_p (wmin0, 0, sgn) && wi::le_p (wmax0, max - (-nm), sgn));
> +}
> diff --git a/gcc/match.pd b/gcc/match.pd
> index ceae1c34abc..1aaa5530577 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -881,6 +881,28 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>  #endif
>     ))))
>  
> +#if GIMPLE
> +/* Simplify ((t + -N*M) / N + M) -> t / N.  */
> +(for div (trunc_div exact_div)
> + (simplify
> +  (plus (div (plus @0 INTEGER_CST@1) INTEGER_CST@2) INTEGER_CST@3)
> +  (with {wide_int n = wi::to_wide (@2); wide_int m = wi::to_wide (@3);}
> +    (if (INTEGRAL_TYPE_P (type)
> +	 && n * m == -wi::to_wide (@1)
> +	 && optimize_x_minus_NM_div_N_plus_M (@0, n, m, type))
> +    (div @0 @2)))))
> +
> +/* Simplify ((t + -(M<<N)) >> N + M) -> t >> N.  */
> +(simplify
> + (plus (rshift (plus @0 INTEGER_CST@1) INTEGER_CST@2) INTEGER_CST@3)
> + (with {wide_int n = wi::to_wide (@2); wide_int m = wi::to_wide (@3);}
> +   (if (INTEGRAL_TYPE_P (type)
> +	&& (m << n) == -wi::to_wide (@1)
> +	&& optimize_x_minus_NM_div_N_plus_M (@0,
> +	     wi::one (TYPE_PRECISION (type)) << n, m, type))
> +    (rshift @0 @2))))
> +#endif
> +
>  (for op (negate abs)
>   /* Simplify cos(-x) and cos(|x|) -> cos(x).  Similarly for cosh.  */
>   (for coss (COS COSH)
> diff --git a/gcc/testsuite/gcc.dg/pr108757-1.c b/gcc/testsuite/gcc.dg/pr108757-1.c
> new file mode 100644
> index 00000000000..349318a7c82
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr108757-1.c
> @@ -0,0 +1,17 @@
> +/* PR tree-optimization/108757 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +#include <limits.h>
> +#define N 4
> +#define M 3
> +#define GAP 0
> +typedef unsigned int UINT;
> +typedef int INT;
> +#define UMAX UINT_MAX
> +#define IMAX INT_MAX
> +#define IMIN INT_MIN
> +#include "pr108757.h"
> +
> +/* { dg-final { scan-tree-dump-not " = x_\[0-9\]\\(D\\) \\+ " "optimized" } } */
> +/* { dg-final { scan-tree-dump-not " = b_\[0-9\] \\+ " "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/pr108757-2.c b/gcc/testsuite/gcc.dg/pr108757-2.c
> new file mode 100644
> index 00000000000..7302e9c606c
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr108757-2.c
> @@ -0,0 +1,18 @@
> +/* PR tree-optimization/108757 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +#include <limits.h>
> +#define N 4
> +#define M 3
> +#define GAP 2
> +typedef unsigned int UINT;
> +typedef int INT;
> +#define UMAX UINT_MAX
> +#define IMAX INT_MAX
> +#define IMIN INT_MIN
> +#include "pr108757.h"
> +
> +/* { dg-final { scan-tree-dump-times " = x_\[0-9\]\\(D\\) \\+ " 16 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times " = b_\[0-9\] \\+ " 16 "optimized" } } */
> +
> diff --git a/gcc/testsuite/gcc.dg/pr108757.h b/gcc/testsuite/gcc.dg/pr108757.h
> new file mode 100644
> index 00000000000..9e12d106aad
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr108757.h
> @@ -0,0 +1,160 @@
> +#define NOINLINE __attribute__ ((noinline))
> +UINT NOINLINE
> +opt_u1 (UINT x)
> +{
> +  if (x < (M * N) - GAP)
> +    return 0;
> +  UINT a = x - (M * N);
> +  UINT b = a / N;
> +  return b + M;
> +}
> +
> +UINT NOINLINE
> +opt_u2 (UINT x)
> +{
> +  if (x > (UMAX - (M * N) + GAP))
> +    return 0;
> +  UINT a = x + (M * N);
> +  UINT b = a / N;
> +  return b - M;
> +}
> +
> +INT NOINLINE
> +opt_s1 (INT x)
> +{
> +  if (x < (M * N) - GAP)
> +    return 0;
> +  INT a = x - (M * N);
> +  INT b = a / N;
> +  return b + M;
> +}
> +
> +INT NOINLINE
> +opt_s2 (INT x)
> +{
> +  if (x < IMIN + (M * N) - GAP || x > 0)
> +    return 0;
> +  INT a = x - (M * N);
> +  INT b = a / N;
> +  return b + M;
> +}
> +
> +INT NOINLINE
> +opt_s3 (INT x)
> +{
> +  if (x < (M * N) - GAP)
> +    return 0;
> +  INT a = x - (M * N);
> +  INT b = a / -N;
> +  return b + -M;
> +}
> +
> +INT NOINLINE
> +opt_s4 (INT x)
> +{
> +  if (x < IMIN + (M * N) - GAP || x > 0)
> +    return 0;
> +  INT a = x - (M * N);
> +  INT b = a / -N;
> +  return b + -M;
> +}
> +
> +INT NOINLINE
> +opt_s5 (INT x)
> +{
> +  if (x > (-M * N) + GAP)
> +    return 0;
> +  INT a = x - (-M * N);
> +  INT b = a / N;
> +  return b + -M;
> +}
> +
> +INT NOINLINE
> +opt_s6 (INT x)
> +{
> +  if (x > IMAX - (M * N) + GAP || x < 0)
> +    return 0;
> +  INT a = x - (-M * N);
> +  INT b = a / N;
> +  return b + -M;
> +}
> +
> +INT NOINLINE
> +opt_s7 (INT x)
> +{
> +  if (x > (M * -N) + GAP)
> +    return 0;
> +  INT a = x - (M * -N);
> +  INT b = a / -N;
> +  return b + M;
> +}
> +
> +INT NOINLINE
> +opt_s8 (INT x)
> +{
> +  if (x > IMAX - (M * N) + GAP || x < 0)
> +    return 0;
> +  INT a = x - (M * -N);
> +  INT b = a / -N;
> +  return b + M;
> +}
> +
> +UINT NOINLINE
> +opt_u3 (UINT x)
> +{
> +  if (x < (M << N) - GAP)
> +    return 0;
> +  UINT a = x - (M << N);
> +  UINT b = a >> N;
> +  return b + M;
> +}
> +
> +UINT NOINLINE
> +opt_u4 (UINT x)
> +{
> +  if (x > (UMAX - (M << N)) + GAP)
> +    return 0;
> +  UINT a = x + (M << N);
> +  UINT b = a >> N;
> +  return b - M;
> +}
> +
> +INT NOINLINE
> +opt_s9 (INT x)
> +{
> +  if (x < (M << N) - GAP)
> +    return 0;
> +  INT a = x - (M << N);
> +  INT b = a >> N;
> +  return b + M;
> +}
> +
> +INT NOINLINE
> +opt_s10 (INT x)
> +{
> +  if (x < IMIN + (M << N) - GAP || x > 0)
> +    return 0;
> +  INT a = x - (M << N);
> +  INT b = a >> N;
> +  return b + M;
> +}
> +
> +INT NOINLINE
> +opt_s11 (INT x)
> +{
> +  if (x > (-M << N) + GAP)
> +    return 0;
> +  INT a = x - (-M << N);
> +  INT b = a >> N;
> +  return b + -M;
> +}
> +
> +INT NOINLINE
> +opt_s12 (INT x)
> +{
> +  if (x > IMAX - (M << N) + GAP || x < 0)
> +    return 0;
> +  INT a = x - (-M << N);
> +  INT b = a >> N;
> +  return b + -M;
> +}
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
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