| Message ID | alpine.LNX.2.00.1311211002490.8615@zhemvz.fhfr.qr |
|---|---|
| State | New |
| Headers | show |
Richard Biener <rguenther@suse.de> writes: > This patch decrements nb_iterations_upper_bound by one after we copied > the loop header. This allows niter + 1 to more often not overflow. > > Bootstrapped and tested on x86_64-unknown-linux-gnu, installed to trunk. > > Richard. > > 2013-11-21 Richard Biener <rguenther@suse.de> > > * tree-ssa-loop-ch.c (copy_loop_headers): Decrement > nb_iterations_upper_bound by one. This seems to regress the following testcase, compiled at -O2 on x86_64-linux-gnu. Thanks, Richard void __attribute__ ((noinline)) f (long *s, long *t, int len1, int len2) { int i, j; j = 2; for (i = len1 - 1; i >= 0; i--) { s[j--] = (i < len2 ? t[i] : t[len2 - 1] < 0 ? -1 : 0); if (j < 0) break; } } long s[3]; long t[3]; int main (void) { t[0] = 1; t[1] = 2; t[2] = 3; f (s, t, 3, 3); if (memcmp (s, t, sizeof (s)) != 0) abort (); return 0; }
On Fri, 22 Nov 2013, Richard Sandiford wrote: > Richard Biener <rguenther@suse.de> writes: > > This patch decrements nb_iterations_upper_bound by one after we copied > > the loop header. This allows niter + 1 to more often not overflow. > > > > Bootstrapped and tested on x86_64-unknown-linux-gnu, installed to trunk. > > > > Richard. > > > > 2013-11-21 Richard Biener <rguenther@suse.de> > > > > * tree-ssa-loop-ch.c (copy_loop_headers): Decrement > > nb_iterations_upper_bound by one. > > This seems to regress the following testcase, compiled at -O2 on > x86_64-linux-gnu. Thanks for the testcase, I have reverted the patch and installed the testcase. Thanks, Richard. > Thanks, > Richard > > > void __attribute__ ((noinline)) > f (long *s, long *t, int len1, int len2) > { > int i, j; > > j = 2; > for (i = len1 - 1; i >= 0; i--) > { > s[j--] = (i < len2 ? t[i] : t[len2 - 1] < 0 ? -1 : 0); > if (j < 0) > break; > } > } > > long s[3]; > long t[3]; > > int > main (void) > { > t[0] = 1; > t[1] = 2; > t[2] = 3; > f (s, t, 3, 3); > if (memcmp (s, t, sizeof (s)) != 0) > abort (); > return 0; > }
Index: gcc/tree-ssa-loop-ch.c =================================================================== --- gcc/tree-ssa-loop-ch.c (revision 205097) +++ gcc/tree-ssa-loop-ch.c (working copy) @@ -243,6 +243,16 @@ copy_loop_headers (void) are not now, since there was the loop exit condition. */ split_edge (loop_preheader_edge (loop)); split_edge (loop_latch_edge (loop)); + + /* We peeled off one iteration of the loop thus we can lower + the maximum number of iterations if we have a previously + recorded value for that. */ + double_int max; + if (get_max_loop_iterations (loop, &max)) + { + max -= double_int_one; + loop->nb_iterations_upper_bound = max; + } } update_ssa (TODO_update_ssa);