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[committed] [PR tree-optimization/86010] More aggressively trim partially dead mem* and str* calls
- From: Jeff Law <law at redhat dot com>
- To: gcc-patches <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 5 Jul 2018 21:44:10 -0600
- Subject: [committed] [PR tree-optimization/86010] More aggressively trim partially dead mem* and str* calls
As noted in BZ 86010 we can be more aggressive when trimming tails of
mem* or str* calls in gimple DSE since trimming a tail doesn't affect
alignment and residuals are usually handled pretty efficiently in libc.
Additionally, if the total number of live bytes left is smaller than a
word, then it's highly likely we'll open-code the mem* or str* routine.
So we allow more aggressive trimming in that case too.
What's left to be able to close out 86010 is to identify when a memory
store could be merged with a subsequent memset. I'm skeptical of the
importance of that optimization, though perhaps it comes up often enough
with structure initializations to be worth doing.
Bootstrapped and regression tested on x86_64-linux-gnu. Installing on
* tree-ssa-dse.c (compute_trims): More aggressively trim at
both the head and tail of mem* and str* calls.
diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
index 1af50a0..ebc4a1e 100644
@@ -240,11 +240,14 @@ compute_trims (ao_ref *ref, sbitmap live, int *trim_head, int *trim_tail,
/* Now identify how much, if any of the tail we can chop off. */
+ int last_live = bitmap_last_set_bit (live);
if (ref->size.is_constant (&const_size))
int last_orig = (const_size / BITS_PER_UNIT) - 1;
- int last_live = bitmap_last_set_bit (live);
- *trim_tail = (last_orig - last_live) & ~0x1;
+ /* We can leave inconvenient amounts on the tail as
+ residual handling in mem* and str* functions is usually
+ reasonably efficient. */
+ *trim_tail = last_orig - last_live;
*trim_tail = 0;
@@ -252,7 +255,12 @@ compute_trims (ao_ref *ref, sbitmap live, int *trim_head, int *trim_tail,
/* Identify how much, if any of the head we can chop off. */
int first_orig = 0;
int first_live = bitmap_first_set_bit (live);
- *trim_head = (first_live - first_orig) & ~0x1;
+ *trim_head = first_live - first_orig;
+ /* If more than a word remains, then make sure to keep the
+ starting point at least word aligned. */
+ if (last_live - first_live > UNITS_PER_WORD)
+ *trim_head &= (UNITS_PER_WORD - 1);
if ((*trim_head || *trim_tail)
&& dump_file && (dump_flags & TDF_DETAILS))