https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114187
--- Comment #5 from GCC 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:d35b5b0e0a0727cfdaba5f859e44116c33648639 commit r14-9287-gd35b5b0e0a0727cfdaba5f859e44116c33648639 Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Mon Mar 4 00:47:08 2024 +0000 PR target/114187: Fix ?Fmode SUBREG simplification in simplify_subreg. This patch fixes PR target/114187 a typo/missed-optimization in simplify-rtx that's exposed by (my) changes to x86_64's parameter passing. The context is that construction of double word (TImode) values now uses the idiom: (ior:TI (ashift:TI (zero_extend:TI (reg:DI x)) (const_int 64 [0x40])) (zero_extend:TI (reg:DI y))) Extracting the DImode highpart and lowpart halves of this complex expression is supported by simplications in simplify_subreg. The problem is when the doubleword TImode value represents two DFmode fields, there isn't a direct simplification to extract the highpart or lowpart SUBREGs, but instead GCC uses two steps, extract the DImode {high,low} part and then cast the result back to a floating point mode, DFmode. The (buggy) code to do this is: /* If the outer mode is not integral, try taking a subreg with the equivalent integer outer mode and then bitcasting the result. Other simplifications rely on integer to integer subregs and we'd potentially miss out on optimizations otherwise. */ if (known_gt (GET_MODE_SIZE (innermode), GET_MODE_SIZE (outermode)) && SCALAR_INT_MODE_P (innermode) && !SCALAR_INT_MODE_P (outermode) && int_mode_for_size (GET_MODE_BITSIZE (outermode), 0).exists (&int_outermode)) { rtx tem = simplify_subreg (int_outermode, op, innermode, byte); if (tem) return simplify_gen_subreg (outermode, tem, int_outermode, byte); } The issue/mistake is that the second call, to simplify_subreg, shouldn't use "byte" as the final argument; the offset has already been handled by the first call, to simplify_subreg, and this second call is just a type conversion from an integer mode to floating point (from DImode to DFmode). Interestingly, this mistake was already spotted by Richard Sandiford when the optimization was originally contributed in January 2023. https://gcc.gnu.org/pipermail/gcc-patches/2023-January/610920.html >> Richard Sandiford writes: >> Also, the final line should pass 0 rather than byte. Unfortunately a miscommunication/misunderstanding in a later thread https://gcc.gnu.org/pipermail/gcc-patches/2023-February/612898.html resulted in this correction being undone. Using lowpart_subreg should avoid/reduce confusion in future. 2024-03-03 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR target/114187 * simplify-rtx.cc (simplify_context::simplify_subreg): Call lowpart_subreg to perform type conversion, to avoid confusion over the offset to use in the call to simplify_reg_subreg. gcc/testsuite/ChangeLog PR target/114187 * g++.target/i386/pr114187.C: New test case.
