[Bug target/109972] New: RISC-V: Could use umodsi3/udivsi3/divsi3 libcalls for 32-bit division/remainder on RV64 without M extension

[Bug target/109972] New: RISC-V: Could use umodsi3/udivsi3/divsi3 libcalls for 32-bit division/remainder on RV64 without M extension

[craig.topper at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109972

            Bug ID: 109972
           Summary: RISC-V: Could use umodsi3/udivsi3/divsi3 libcalls for
                    32-bit division/remainder on RV64 without M extension
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: craig.topper at gmail dot com
  Target Milestone: ---

There's an opportunity to improve code size for 32-bit division and remainder
on RV64 without the M extension.

Currently gcc calls the umoddi3/udivdi3/divdi3 functions by zero/sign extending
the operands. In the case of the unsigned functions this requires two shifts to
zero the upper bits. For signed, it's two sext.w if the operands are not
already sign extended.

All 3 functions are followed by a sext.w if the result needs be sign extended.

libgcc contains umodsi3/udivsi3/divsi3 functions that handle the zero extending
of inputs and sign extending the result. Internally they call the di3 functions
to do the computation. These functions could be used to reduce code size at the
caller.

There is no signed modsi3 function in libgcc. Probably because umoddi3(sext(X),
sext(Y)) is guaranteed to produce a result that is sign extended. gcc seems to
not know this and still emits a sext.w after the call to umoddi3.

godbolt https://godbolt.org/z/ax3Khc6cM

unsigned divu(unsigned x, unsigned y) {
  return x / y;
}

unsigned remu(unsigned x, unsigned y) {
  return x % y;
}

int div(int x, int y) {
  return x / y;
}

int rem(int x, int y) {
  return x % y;
}

[Bug target/109972] RISC-V: Could use umodsi3/udivsi3/divsi3 libcalls for 32-bit division/remainder on RV64 without M extension

[palmer at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109972

palmer at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |palmer at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-06-01
           Severity|normal                      |enhancement

--- Comment #1 from palmer at gcc dot gnu.org ---
Thanks.  Craig and I had talked about this offline, it looks like a real
improvement to me.  We're not super worried about rv32 or code size, maybe Kito
is?

[Bug target/109972] RISC-V: Could use umodsi3/udivsi3/divsi3 libcalls for 32-bit division/remainder on RV64 without M extension

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109972

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to palmer from comment #1)
> Thanks.  Craig and I had talked about this offline, it looks like a real
> improvement to me.  We're not super worried about rv32 or code size, maybe
> Kito is?

I am care about RV32 code quality though the core I am working with has the M
extension so this case is not as important to me.

[Bug target/109972] RISC-V: Could use umodsi3/udivsi3/divsi3 libcalls for 32-bit division/remainder on RV64 without M extension

[kito at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109972

--- Comment #3 from Kito Cheng <kito at gcc dot gnu.org> ---
We care but it's lower priority compare to other configuration, so create bug
to tracking here should be best solution for now :P