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[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
the trunk.

	PR tree-optimization/86010
	* 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
--- a/gcc/tree-ssa-dse.c
+++ b/gcc/tree-ssa-dse.c
@@ -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.  */
   HOST_WIDE_INT const_size;
+  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))

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