On Wed, 24 Nov 2021, Jakub Jelinek wrote:
> 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?
OK.
> 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?
Not sure if it is worth the trouble - the XOR handling sounds
straight forward at least. But sure, the merging routine could
simply be conservatively correct here.
Thanks,
Richard.
> 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
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Ivo Totev; HRB 36809 (AG Nuernberg)
