On Mon, Nov 22, 2021 at 08:39:42AM -0000, Roger Sayle wrote:
> This patch implements PR tree-optimization/103345 to merge adjacent
> loads when combined with addition or bitwise xor. The current code
> in gimple-ssa-store-merging.c's find_bswap_or_nop alreay handles ior,
> so that all that's required is to treat PLUS_EXPR and BIT_XOR_EXPR in
> the same way at BIT_IOR_EXPR.
Unfortunately they aren't exactly the same. They work the same if always
at least one operand (or corresponding byte in it) is known to be 0,
0 | 0 = 0 ^ 0 = 0 + 0 = 0. But for | also x | x = x for any other x,
so perform_symbolic_merge has been accepting either that at least one
of the bytes is 0 or that both are the same, but that is wrong for ^
and +.
The following patch fixes that by passing through the code of binary
operation and allowing non-zero masked1 == masked2 through only
for BIT_IOR_EXPR.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Thinking more about it, perhaps we could do more for BIT_XOR_EXPR.
We could allow masked1 == masked2 case for it, but would need to
do something different than the
n->n = n1->n | n2->n;
we do on all the bytes together.
In particular, for masked1 == masked2 if masked1 != 0 (well, for 0
both variants are the same) and masked1 != 0xff we would need to
clear corresponding n->n byte instead of setting it to the input
as x ^ x = 0 (but if we don't know what x and y are, the result is
also don't know). Now, for plus it is much harder, because not only
for non-zero operands we don't know what the result is, but it can
modify upper bytes as well. So perhaps only if current's byte
masked1 && masked2 set the resulting byte to 0xff (unknown) iff
the byte above it is 0 and 0, and set that resulting byte to 0xff too.
Also, even for | we could instead of return NULL just set the resulting
byte to 0xff if it is different, perhaps it will be masked off later on.
Ok to handle that incrementally?
2021-11-24 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/103376
* gimple-ssa-store-merging.c (perform_symbolic_merge): Add CODE
argument. If CODE is not BIT_IOR_EXPR, ensure that one of masked1
or masked2 is 0.
(find_bswap_or_nop_1, find_bswap_or_nop,
imm_store_chain_info::try_coalesce_bswap): Adjust
perform_symbolic_merge callers.
* gcc.c-torture/execute/pr103376.c: New test.
--- gcc/gimple-ssa-store-merging.c.jj 2021-11-23 10:26:30.000000000 +0100
+++ gcc/gimple-ssa-store-merging.c 2021-11-23 11:49:33.806168782 +0100
@@ -434,14 +434,14 @@ find_bswap_or_nop_load (gimple *stmt, tr
return true;
}
-/* Compute the symbolic number N representing the result of a bitwise OR on 2
- symbolic number N1 and N2 whose source statements are respectively
- SOURCE_STMT1 and SOURCE_STMT2. */
+/* Compute the symbolic number N representing the result of a bitwise OR,
+ bitwise XOR or plus on 2 symbolic number N1 and N2 whose source statements
+ are respectively SOURCE_STMT1 and SOURCE_STMT2. CODE is the operation. */
gimple *
perform_symbolic_merge (gimple *source_stmt1, struct symbolic_number *n1,
gimple *source_stmt2, struct symbolic_number *n2,
- struct symbolic_number *n)
+ struct symbolic_number *n, enum tree_code code)
{
int i, size;
uint64_t mask;
@@ -563,7 +563,9 @@ perform_symbolic_merge (gimple *source_s
masked1 = n1->n & mask;
masked2 = n2->n & mask;
- if (masked1 && masked2 && masked1 != masked2)
+ /* For BIT_XOR_EXPR or PLUS_EXPR, at least one of masked1 and masked2
+ has to be 0, for BIT_IOR_EXPR x | x is still x. */
+ if (masked1 && masked2 && (code != BIT_IOR_EXPR || masked1 != masked2))
return NULL;
}
n->n = n1->n | n2->n;
@@ -769,7 +771,8 @@ find_bswap_or_nop_1 (gimple *stmt, struc
return NULL;
source_stmt
- = perform_symbolic_merge (source_stmt1, &n1, source_stmt2, &n2, n);
+ = perform_symbolic_merge (source_stmt1, &n1, source_stmt2, &n2, n,
+ code);
if (!source_stmt)
return NULL;
@@ -943,7 +946,8 @@ find_bswap_or_nop (gimple *stmt, struct
else if (!do_shift_rotate (LSHIFT_EXPR, &n0, eltsz))
return NULL;
ins_stmt
- = perform_symbolic_merge (ins_stmt, &n0, source_stmt, &n1, n);
+ = perform_symbolic_merge (ins_stmt, &n0, source_stmt, &n1, n,
+ BIT_IOR_EXPR);
if (!ins_stmt)
return NULL;
@@ -2881,7 +2885,7 @@ imm_store_chain_info::try_coalesce_bswap
end = MAX (end, info->bitpos + info->bitsize);
ins_stmt = perform_symbolic_merge (ins_stmt, &n, info->ins_stmt,
- &this_n, &n);
+ &this_n, &n, BIT_IOR_EXPR);
if (ins_stmt == NULL)
return false;
}
--- gcc/testsuite/gcc.c-torture/execute/pr103376.c.jj 2021-11-23
12:03:38.339948150 +0100
+++ gcc/testsuite/gcc.c-torture/execute/pr103376.c 2021-11-23
12:02:44.668723595 +0100
@@ -0,0 +1,29 @@
+/* PR tree-optimization/103376 */
+
+long long a = 0x123456789abcdef0LL, f;
+int b, c, *d;
+
+__attribute__((noipa)) void
+foo (int x)
+{
+ asm volatile ("" : : "r" (x));
+}
+
+int
+main ()
+{
+ long long e;
+ e = a;
+ if (b)
+ {
+ foo (c);
+ d = (int *) 0;
+ while (*d)
+ ;
+ }
+ f = a ^ e;
+ asm volatile ("" : "+m" (f));
+ if (f != 0)
+ __builtin_abort ();
+ return 0;
+}
Jakub
