[Bug target/123838] New: riscv: Optimize int shift left by (31 - n).

[rdapp at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123838

            Bug ID: 123838
           Summary: riscv: Optimize int shift left by (31 - n).
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: easyhack, missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rdapp at gcc dot gnu.org
                CC: law at gcc dot gnu.org
  Target Milestone: ---
            Target: riscv

Similar to PR123792 for aarch64, we could do better at

 int f1 (int x, int n)
 {
   return x << (31 - n);
 }

and related.

Currently:
        li      a5,31
        subw    a5,a5,a1
        sllw    a0,a0,a5
        ret

clang:
        not     a1, a1
        sllw    a0, a0, a1
        ret

It feels like we should already have a similar bug but I didn't find one.