https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101403
--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Roger Sayle <sa...@gcc.gnu.org>: https://gcc.gnu.org/g:5f5fbb550af7d9d6cb56ae8f607fea0eccaa9295 commit r12-2238-g5f5fbb550af7d9d6cb56ae8f607fea0eccaa9295 Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Mon Jul 12 08:24:27 2021 +0100 PR tree-optimization/101403: Incorrect folding of ((T)bswap(x))>>C My sincere apologies for the breakage. My recent patch to fold bswapN(x)>>C where the constant C was large enough that the result only contains bits from the low byte, and can therefore avoid the byte swap contains a minor logic error. The pattern contains a convert? allowing an extension to occur between the bswap and the shift. The logic is correct if there's no extension, or the extension has the same sign as the shift, but I'd mistakenly convinced myself that these couldn't have different signedness. (T)bswap16(x)>>12 is (T)((unsigned char)x>>4) or (T)((signed char)x>>4). The bug is that for zero-extensions to signed type T, we need to use the unsigned char variant [the signedness of the byte shift is not (always) the same as the signedness of T and the original shift]. Then because I'm now paranoid, I've also added a clause to handle the hypothetical (but in practice impossible) sign-extension to an unsigned type T, which can implemented as (T)(x<<8)>>12. 2021-07-12 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR tree-optimization/101403 * match.pd ((T)bswap(X)>>C): Correctly handle cases where signedness of the shift is not the same as the signedness of the type extension. gcc/testsuite/ChangeLog PR tree-optimization/101403 * gcc.dg/pr101403.c: New test case.
