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bswap: Fix up symbolic merging for xor and plus [PR103376]

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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.

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.

2021-11-24  Jakub Jelinek  <jakub@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.
This commit is contained in:
Jakub Jelinek 2021-11-24 09:54:44 +01:00
parent 52554dde7b
commit 04eccbbe3d
2 changed files with 41 additions and 8 deletions
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20
gcc/gimple-ssa-store-merging.c
View file

@ -434,14 +434,14 @@ find_bswap_or_nop_load (gimple *stmt, tree ref, struct symbolic_number *n)
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_stmt1, struct symbolic_number *n1,
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, struct symbolic_number *n, int limit)
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 symbolic_number *n, bool *bswap,
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 (merged_store_group *merged_store,
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;
}

29
gcc/testsuite/gcc.c-torture/execute/pr103376.c Normal file
View file

@ -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;
}