https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106585
--- Comment #9 from Andrew Waterman <andrew at sifive dot com> --- On Wed, Dec 7, 2022 at 7:02 PM palmer at gcc dot gnu.org via Gcc-bugs <gcc-bugs@gcc.gnu.org> wrote: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106585 > > palmer at gcc dot gnu.org changed: > > What |Removed |Added > ---------------------------------------------------------------------------- > CC| |palmer at gcc dot gnu.org > > --- Comment #8 from palmer at gcc dot gnu.org --- > (In reply to Jeffrey A. Law from comment #7) > > Raphael and I are poking at this a bit. I can't convince myself that it's > > actually safe to use GPR for the bit manipulation patterns. > > > > For rv64 I'm pretty sure the b* instructions are operating on 64bit > > quantities only. Meaning they might twiddle the SI sign bit without > > extending. If we were to change these patterns to use GPR and the result > > then fed an addw (for example) then we would have inconsistent register > > state as operand twiddled by the prior b* pattern wouldn't have been sign > > extended. > > > > To be clear, I think this is a limitation imposed by the ISA docs, not GCC > > where this will be reasonably well defined. > > So you're worried about addw (and the various other OP-32 instructions) > needing > signed extended high parts in registers in order to function as expected? > I've > never gotten that from the ISA manual, there might be some vestigial MIPS-isms > floating around the RISC-V port that indicate that though (as we've got > similar > constraints for the comparisons). > > That said, I'v gone and actually read the ISA manual here and it's not at all > specific. I'm seeing > > ADDW and SUBW are RV64I-only instructions that are defined analogously > to ADD and SUB but operate on 32-bit values and produce signed 32-bit > results. Overflows are ignored, and the low 32-bits of the result is > sign-extended to 64-bits and written to the destination register. > > which doesn't explicitly say the high 32-bits of the inputs are ignored. As > far as I can tell "32-bit values" isn't defined anywhere, so that's not so > useful. > > Do you know if there's any hardware that needs extended values for addw and > friends? That'd almost certainly break a lot of binaries, but I could > certainly buy an argument saying it's to the spec (and the actual words in the > spec, not just this "anything goes" compatibility stuff). The spec explicitly says that the upper 32 bits of the inputs are ignored; you just need to read a few paragraphs up. https://github.com/riscv/riscv-isa-manual/blob/b7080e0d18765730ff4f3d07b866b4884a8be401/src/rv64.tex#L18-L21 > > > With that in mind I think the only path forward is new patterns that (sadly) > > use explicit subregs for sources, but still set a DImode destination. > > > > I'm the newbie here, so if I've misinterpreted the ISA docs incorrectly, > > don't hesitate to let me know. > > Kind of just a related FYI: the comparison instructions and various bits of > the > ABI do require values in canonical forms (the ABI stuff isn't exactly sign > extended, but there's a rule). That's all a big fragile.
