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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. This patch just punts on plus if both corresponding bytes are non-zero, otherwise implements the above. 2021-11-25 Jakub Jelinek <jakub@redhat.com> PR tree-optimization/103376 * gimple-ssa-store-merging.c (perform_symbolic_merge): For BIT_IOR_EXPR, if masked1 && masked2 && masked1 != masked2, don't punt, but set the corresponding result byte to MARKER_BYTE_UNKNOWN. For BIT_XOR_EXPR similarly and if masked1 == masked2 and the byte isn't MARKER_BYTE_UNKNOWN, set the corresponding result byte to 0. * gcc.dg/optimize-bswapsi-7.c: New test. |
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