https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71775
--- Comment #3 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:c23a9c87cc62bd177fd0d4db6ad34b34e1b9a31f commit r13-1942-gc23a9c87cc62bd177fd0d4db6ad34b34e1b9a31f Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Wed Aug 3 08:55:35 2022 +0100 Some additional zero-extension related optimizations in simplify-rtx. This patch implements some additional zero-extension and sign-extension related optimizations in simplify-rtx.cc. The original motivation comes from PR rtl-optimization/71775, where in comment #2 Andrew Pinksi sees: Failed to match this instruction: (set (reg:DI 88 [ _1 ]) (sign_extend:DI (subreg:SI (ctz:DI (reg/v:DI 86 [ x ])) 0))) On many platforms the result of DImode CTZ is constrained to be a small unsigned integer (between 0 and 64), hence the truncation to 32-bits (using a SUBREG) and the following sign extension back to 64-bits are effectively a no-op, so the above should ideally (often) be simplified to "(set (reg:DI 88) (ctz:DI (reg/v:DI 86 [ x ]))". To implement this, and some closely related transformations, we build upon the existing val_signbit_known_clear_p predicate. In the first chunk, nonzero_bits knows that FFS and ABS can't leave the sign-bit bit set, so the simplification of of ABS (ABS (x)) and ABS (FFS (x)) can itself be simplified. The second transformation is that we can canonicalized SIGN_EXTEND to ZERO_EXTEND (as in the PR 71775 case above) when the operand's sign-bit is known to be clear. The final two chunks are for SIGN_EXTEND of a truncating SUBREG, and ZERO_EXTEND of a truncating SUBREG respectively. The nonzero_bits of a truncating SUBREG pessimistically thinks that the upper bits may have an arbitrary value (by taking the SUBREG), so we need look deeper at the SUBREG's operand to confirm that the high bits are known to be zero. Unfortunately, for PR rtl-optimization/71775, ctz:DI on x86_64 with default architecture options is undefined at zero, so we can't be sure the upper bits of reg:DI 88 will be sign extended (all zeros or all ones). nonzero_bits knows this, so the above transformations don't trigger, but the transformations themselves are perfectly valid for other operations such as FFS, POPCOUNT and PARITY, and on other targets/-march settings where CTZ is defined at zero. 2022-08-03 Roger Sayle <ro...@nextmovesoftware.com> Segher Boessenkool <seg...@kernel.crashing.org> Richard Sandiford <richard.sandif...@arm.com> gcc/ChangeLog * simplify-rtx.cc (simplify_unary_operation_1) <ABS>: Add optimizations for CLRSB, PARITY, POPCOUNT, SS_ABS and LSHIFTRT that are all positive to complement the existing FFS and idempotent ABS simplifications. <SIGN_EXTEND>: Canonicalize SIGN_EXTEND to ZERO_EXTEND when val_signbit_known_clear_p is true of the operand. Simplify sign extensions of SUBREG truncations of operands that are already suitably (zero) extended. <ZERO_EXTEND>: Simplify zero extensions of SUBREG truncations of operands that are already suitably zero extended.
