https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84958
Bug ID: 84958
Summary: int loads not eliminated against larger stores
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vries at gcc dot gnu.org
Target Milestone: ---
[ As discussed here: https://gcc.gnu.org/ml/gcc-patches/2018-03/msg00800.html ]
This test-case:
...
int foo()
{
int a[10];
for(int i = 0; i < 10; ++i)
a[i] = i*i;
int res = 0;
for(int i = 0; i < 10; ++i)
res += a[i];
return res;
}
...
compiled with -O3 results in this gimple at optimized:
...
MEM[(int *)&a] = { 0, 1 };
MEM[(int *)&a + 8B] = { 4, 9 };
MEM[(int *)&a + 16B] = { 16, 25 };
MEM[(int *)&a + 24B] = { 36, 49 };
MEM[(int *)&a + 32B] = { 64, 81 };
_6 = a[0];
_28 = a[1];
res_29 = _6 + _28;
_35 = a[2];
res_36 = res_29 + _35;
_42 = a[3];
res_43 = res_36 + _42;
_49 = a[4];
res_50 = res_43 + _49;
_56 = a[5];
res_57 = res_50 + _56;
_63 = a[6];
res_64 = res_57 + _63;
_70 = a[7];
res_71 = res_64 + _70;
_77 = a[8];
res_78 = res_71 + _77;
_2 = a[9];
res_11 = _2 + res_78;
a ={v} {CLOBBER};
return res_11;
...
Loop vectorization has no effect, and the scalar loops are completely unrolled.
Then slp vectorization vectorizes the stores.
When disabling slp vectorization, we have instead:
...
return 285;
...
[ FWIW, adding an extra fre pass here also results in optimal gimple:
...
diff --git a/gcc/passes.def b/gcc/passes.def
index 3ebcfc30349..6b64f600c4a 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -325,6 +325,7 @@ along with GCC; see the file COPYING3. If not see
NEXT_PASS (pass_tracer);
NEXT_PASS (pass_thread_jumps);
NEXT_PASS (pass_dominator, false /* may_peel_loop_headers_p */);
+ NEXT_PASS (pass_fre);
NEXT_PASS (pass_strlen);
NEXT_PASS (pass_thread_jumps);
NEXT_PASS (pass_vrp, false /* warn_array_bounds_p */);
...
]
