Excessive use of __builtin_assume_aligned can cause missed optimizations
because those calls are propagation barriers. The following removes
those that are redundant and provide no extra information, on the
testcase allowng store-merging to apply.
Since the bit lattice and the const/copy lattice are merged
we cannot track this during CCP propagation (make a copy out
of the redundant call and propagate that out). Thus I apply
it in the CCP specific folding routine called during
substitute_and_fold only.
Bootstrap and regtest running on x86_64-unknown-linux-gnu.
Richard.
2019-08-20 Richard Biener <rguent...@suse.de>
PR tree-optimization/91482
* tree-ssa-ccp.c (ccp_folder::fold_stmt): Remove useless
BUILT_IN_ASSUME_ALIGNED calls.
* gcc.dg/tree-ssa/pr91482.c: New testcase.
Index: gcc/tree-ssa-ccp.c
===================================================================
--- gcc/tree-ssa-ccp.c (revision 274670)
+++ gcc/tree-ssa-ccp.c (working copy)
@@ -2315,6 +2315,27 @@ ccp_folder::fold_stmt (gimple_stmt_itera
}
}
+ /* If there's no extra info from an assume_aligned call,
+ drop it so it doesn't act as otherwise useless dataflow
+ barrier. */
+ if (gimple_call_builtin_p (stmt, BUILT_IN_ASSUME_ALIGNED))
+ {
+ tree ptr = gimple_call_arg (stmt, 0);
+ ccp_prop_value_t ptrval = get_value_for_expr (ptr, true);
+ ccp_prop_value_t val = bit_value_assume_aligned (stmt, NULL_TREE,
+ ptrval, false);
+ unsigned int ptralign = least_bit_hwi (ptrval.mask.to_uhwi ());
+ unsigned int align = least_bit_hwi (val.mask.to_uhwi ());
+ if (ptralign == align
+ && ((TREE_INT_CST_LOW (ptrval.value) & (align - 1))
+ == (TREE_INT_CST_LOW (val.value) & (align - 1))))
+ {
+ bool res = update_call_from_tree (gsi, ptr);
+ gcc_assert (res);
+ return true;
+ }
+ }
+
/* Propagate into the call arguments. Compared to replace_uses_in
this can use the argument slot types for type verification
instead of the current argument type. We also can safely
Index: gcc/testsuite/gcc.dg/tree-ssa/pr91482.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/pr91482.c (nonexistent)
+++ gcc/testsuite/gcc.dg/tree-ssa/pr91482.c (working copy)
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-ccp1 -fdump-tree-store-merging" } */
+
+void write64 (void *p)
+{
+ unsigned *p1 = (unsigned *) __builtin_assume_aligned (p, 8);
+ *p1++ = 0;
+ unsigned *p2 = (unsigned *) __builtin_assume_aligned (p1, 4);
+ *p2++ = 1;
+}
+
+/* { dg-final { scan-tree-dump-times "__builtin_assume_aligned" 1 "ccp1" } } */
+/* { dg-final { scan-tree-dump "New sequence of 1 stores to replace old one of
2 stores" "store-merging" { target lp64 } } } */
