http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57999
Bug ID: 57999
Summary: Missed constant propagation into trampolines
Product: gcc
Version: 4.8.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: alexey.tourbin at gmail dot com
Created attachment 30562
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30562&action=edit
mergesort.c
The attached source file is a simplistic mergesort implementation (which
actually works, except for the deliberate assertion failure). Its basic
structure is as follows.
static inline void msort(void *b, size_t n, const size_t s,
int (*cmp)(), void *t)
{
auto void msort_rec(char *b, size_t n);
void msort_rec(char *b, size_t n)
{
/* Copying will be inlined */
assert(__builtin_constant_p(s));
...
/* Recursively sort a1 and a2 */
msort_rec(b1, n1);
msort_rec(b2, n2);
...
/* Merge */
...
if (cmp(b1, b2) <= 0) {
n1--;
memcpy(tmp, b1, s);
b1 += s;
...
}
msort_rec(b, n);
}
When the size of the element (size_t s) is a compile-time constant, the
resulting code would benefit greatly from inlining per-element memcpy() calls.
However, gcc fails to propagate the constant size into the trampoline - the
assertion actually fails, and otherwise the resulting code is roughly 2 times
slower.
