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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. |
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