On Wed, 12 Mar 2025, Richard Sandiford wrote:
> We have long had the fold:
>
> /* Pattern match
> tem = (sizetype) ptr;
> tem = tem & algn;
> tem = -tem;
> ... = ptr p+ tem;
> and produce the simpler and easier to analyze with respect to alignment
> ... = ptr & ~algn; */
>
> But the gimple in gcc.target/aarch64/sve/pr98119.c has a variant in
> which a constant is added before the conversion, giving:
>
> tem = (sizetype) (ptr p+ CST);
> tem = tem & algn;
> tem = -tem;
> ... = ptr p+ tem;
>
> This case is also valid if algn fits within the trailing zero bits
> of CST. Adding CST then has no effect.
>
> Similarly the testcase has:
>
> tem = (sizetype) (ptr p+ CST1);
> tem = tem & algn;
> tem = CST2 - tem;
> ... = ptr p+ tem;
>
> This folds to:
>
> ... = (ptr & ~algn) p+ CST2;
>
> if algn fits within the trailing zero bits of both CST1 and CST2.
>
> An alternative would be:
>
> ... = (ptr p+ CST2) & ~algn;
>
> but I would expect the alignment to be more easily shareable than
> the CST2 addition, given that the CST2 addition wasn't being applied
> by a POINTER_PLUS_EXPR.
OK.
Thanks,
Richard.
> gcc/
> * match.pd: Extend pointer alignment folds so that they handle
> the case where a constant is added before or after the alignment.
>
> gcc/testsuite/
> * gcc.dg/pointer-arith-11.c: New test.
> * gcc.dg/pointer-arith-12.c: Likewise.
> ---
> gcc/match.pd | 27 ++++++++
> gcc/testsuite/gcc.dg/pointer-arith-11.c | 39 ++++++++++++
> gcc/testsuite/gcc.dg/pointer-arith-12.c | 82 +++++++++++++++++++++++++
> 3 files changed, 148 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/pointer-arith-11.c
> create mode 100644 gcc/testsuite/gcc.dg/pointer-arith-12.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 3197d1cac75..0dc74cc86f7 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -3037,6 +3037,33 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> (with { tree algn = wide_int_to_tree (TREE_TYPE (@0), ~wi::to_wide (@1)); }
> (bit_and @0 { algn; })))
>
> +/* Also match cases in which a constant is applied:
> +
> + (1) tem = (sizetype) ptr; ---> tem = (sizetype) ptr + CST;
> + (2) tem = -tem ---> tem = CST - tem;
> +
> + and where "& align" masks only trailing zeros of CST. (1) then has no
> + effect, whereas (2) adds CST to the result. */
> +(simplify
> + (pointer_plus @0 (negate (bit_and (convert (pointer_plus @0 INTEGER_CST@1))
> + INTEGER_CST@2)))
> + (if (tree_int_cst_min_precision (@2, UNSIGNED) <= tree_ctz (@1))
> + (with { tree algn = wide_int_to_tree (TREE_TYPE (@0), ~wi::to_wide (@2)); }
> + (bit_and @0 { algn; }))))
> +(simplify
> + (pointer_plus @0 (minus:s INTEGER_CST@1 (bit_and (convert @0)
> INTEGER_CST@2)))
> + (if (tree_int_cst_min_precision (@2, UNSIGNED) <= tree_ctz (@1))
> + (with { tree algn = wide_int_to_tree (TREE_TYPE (@0), ~wi::to_wide (@2)); }
> + (pointer_plus (bit_and @0 { algn; }) @1))))
> +(simplify
> + (pointer_plus @0 (minus:s INTEGER_CST@1
> + (bit_and (convert (pointer_plus @0 INTEGER_CST@2))
> + INTEGER_CST@3)))
> + (with { auto mask_width = tree_int_cst_min_precision (@3, UNSIGNED); }
> + (if (mask_width <= tree_ctz (@1) && mask_width <= tree_ctz (@2))
> + (with { tree algn = wide_int_to_tree (TREE_TYPE (@0), ~wi::to_wide (@3));
> }
> + (pointer_plus (bit_and @0 { algn; }) @1)))))
> +
> /* Try folding difference of addresses. */
> (simplify
> (minus (convert ADDR_EXPR@0) (convert (pointer_plus @1 @2)))
> diff --git a/gcc/testsuite/gcc.dg/pointer-arith-11.c
> b/gcc/testsuite/gcc.dg/pointer-arith-11.c
> new file mode 100644
> index 00000000000..08a295b9453
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pointer-arith-11.c
> @@ -0,0 +1,39 @@
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +/* { dg-final { scan-tree-dump-times { & -16B?;} 4 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { \+ 16;} 3 "optimized" } } */
> +/* { dg-final { scan-tree-dump-not { & 15;} "optimized" } } */
> +/* { dg-final { scan-tree-dump-not { \+ 96;} "optimized" } } */
> +
> +typedef __UINTPTR_TYPE__ uintptr_t;
> +
> +char *
> +f1 (char *x)
> +{
> + char *y = x + 32;
> + x += -((uintptr_t) y & 15);
> + return x;
> +}
> +
> +char *
> +f2 (char *x)
> +{
> + x += 16 - ((uintptr_t) x & 15);
> + return x;
> +}
> +
> +char *
> +f3 (char *x)
> +{
> + char *y = x + 32;
> + x += 16 - ((uintptr_t) y & 15);
> + return x;
> +}
> +
> +char *
> +f4 (char *x)
> +{
> + char *y = x + 16;
> + x += 16 - ((uintptr_t) y & 15);
> + return x;
> +}
> diff --git a/gcc/testsuite/gcc.dg/pointer-arith-12.c
> b/gcc/testsuite/gcc.dg/pointer-arith-12.c
> new file mode 100644
> index 00000000000..ebdcbd3c967
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pointer-arith-12.c
> @@ -0,0 +1,82 @@
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +/* { dg-final { scan-tree-dump-not { & -16B?;} "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { & 15;} 10 "optimized" } } */
> +
> +typedef __UINTPTR_TYPE__ uintptr_t;
> +
> +char *
> +f1 (char *x)
> +{
> + char *y = x + 97;
> + x += -((uintptr_t) y & 15);
> + return x;
> +}
> +
> +char *
> +f2 (char *x)
> +{
> + char *y = x + 98;
> + x += -((uintptr_t) y & 15);
> + return x;
> +}
> +
> +char *
> +f3 (char *x)
> +{
> + char *y = x + 100;
> + x += -((uintptr_t) y & 15);
> + return x;
> +}
> +
> +char *
> +f4 (char *x)
> +{
> + char *y = x + 104;
> + x += -((uintptr_t) y & 15);
> + return x;
> +}
> +
> +char *
> +f5 (char *x)
> +{
> + x += 1 - ((uintptr_t) x & 15);
> + return x;
> +}
> +
> +char *
> +f6 (char *x)
> +{
> + x += 2 - ((uintptr_t) x & 15);
> + return x;
> +}
> +
> +char *
> +f7 (char *x)
> +{
> + x += 4 - ((uintptr_t) x & 15);
> + return x;
> +}
> +
> +char *
> +f8 (char *x)
> +{
> + x += 8 - ((uintptr_t) x & 15);
> + return x;
> +}
> +
> +char *
> +f9 (char *x)
> +{
> + char *y = x + 8;
> + x += 16 - ((uintptr_t) y & 15);
> + return x;
> +}
> +
> +char *
> +f10 (char *x)
> +{
> + char *y = x + 16;
> + x += 8 - ((uintptr_t) y & 15);
> + return x;
> +}
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
