[Bug tree-optimization/94356] Missed optimisation: useless multiplication generated for pointer comparison
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94356 --- Comment #6 from Marc Glisse --- (In reply to Andrew Pinski from comment #5) > Hmm, the following is worse: That looks like a separate issue. We have fold_comparison for GENERIC, and match.pd has related patterns for integers, or for pointers with ==, but not for pointers with <. Strange, I thought I had added those, possibly together with pointer_diff since the behavior is similar.
[Bug tree-optimization/94356] Missed optimisation: useless multiplication generated for pointer comparison
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94356
Andrew Pinski changed:
What|Removed |Added
Severity|normal |enhancement
--- Comment #5 from Andrew Pinski ---
Hmm, the following is worse:
typedef int t[10];
int f(t *p, long long o) {
t *p1 = p+o;
return p < p1;
}
[Bug tree-optimization/94356] Missed optimisation: useless multiplication generated for pointer comparison
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94356 Richard Biener changed: What|Removed |Added Status|UNCONFIRMED |NEW Ever confirmed|0 |1 Last reconfirmed||2020-03-30 --- Comment #4 from Richard Biener --- I also remember tackling this in pointer_int_sum at some point (also with mixed results). Keep in mind the sizetype requirement for POINTER_PLUS_EXPR which IMHO should change to either ssizetype or [s]sizetype - Bin tried the change to ssizetype with again mixed results ;)
[Bug tree-optimization/94356] Missed optimisation: useless multiplication generated for pointer comparison
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94356 --- Comment #3 from Marc Glisse --- I tried https://gcc.gnu.org/pipermail/gcc-patches/2017-May/475037.html some time ago. Having a different type for the multiplication and the offsetting introduced a lot of NOPs and caused a few regressions (from my notes, pta-ptrarith-3.c and ssa-pre-8.c were just testsuite issues, but there were others). I filed a bug or 2 about those (PR 88926 at least). Then I tried to also make the offsets signed, to be consistent, but that's a much bigger piece and I ran out of time and motivation.
[Bug tree-optimization/94356] Missed optimisation: useless multiplication generated for pointer comparison
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94356
Jakub Jelinek changed:
What|Removed |Added
CC||jakub at gcc dot gnu.org
--- Comment #2 from Jakub Jelinek ---
Given
struct S { char a[37]; };
int f1 (struct S *s, int a) { struct S *sa = s + a; return s < sa; }
int f2 (struct S *s, int a, int b) { struct S *sa = s + a; return sa < s + b; }
int f3 (struct S *s, int a) { struct S *sa = s + a; return s == sa; }
int f4 (struct S *s, int a, int b) { struct S *sa = s + a; return sa == s + b;
}
f3 is optimized in:
/* X + Y < Y is the same as X < 0 when there is no overflow. */
...
/* For equality, this is also true with wrapping overflow. */
...
(simplify
(op:c (nop_convert?@3 (pointer_plus@2 (convert1? @0) @1)) (convert2? @0))
(if (tree_nop_conversion_p (TREE_TYPE (@2), TREE_TYPE (@0))
&& tree_nop_conversion_p (TREE_TYPE (@3), TREE_TYPE (@0))
&& (CONSTANT_CLASS_P (@1) || (single_use (@2) && single_use (@3
(op @1 { build_zero_cst (TREE_TYPE (@1)); }
f4 is optimized too, but not exactly sure where, partly e.g. in
/* For integral types with wrapping overflow and C odd fold
x * C EQ/NE y * C into x EQ/NE y. */
(for cmp (eq ne)
(simplify
(cmp (mult @0 INTEGER_CST@1) (mult @2 @1))
(if (INTEGRAL_TYPE_P (TREE_TYPE (@1))
&& TYPE_OVERFLOW_WRAPS (TREE_TYPE (@0))
&& (TREE_INT_CST_LOW (@1) & 1) != 0)
(cmp @0 @2
For f1/f2 we likely only optimize the ptr + x < ptr + y into x < y at RTL time,
and in any case, the multiplications are done right now in sizetype which is
unsigned type.
[Bug tree-optimization/94356] Missed optimisation: useless multiplication generated for pointer comparison
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94356 --- Comment #1 from Marc Glisse --- That's because internally we use an unsigned type for offsets (including for the multiplication). There have been tries to change that...
