[Bug middle-end/91866] New: Sign extend of an int is not recognized

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91866

            Bug ID: 91866
           Summary: Sign extend of an int is not recognized
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Consider the example:


using size_t = unsigned long long;

size_t index0(int i) {
    return size_t(i + 1) - 1;
}


GCC generates the following assembly:

index0(int):
  lea eax, [rdi+1]
  cdqe
  sub rax, 1
  ret


However a more optimal assembly is possible:

index0(int): # @index0(int)
  movsxd rax, edi
  ret

Godbolt playground: https://godbolt.org/z/3j7_SE