[llvm-bugs] [Bug 43431] New: Value range knowledge of higher bits not used in optimizations

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43431

            Bug ID: 43431
           Summary: Value range knowledge of higher bits not used in
                    optimizations
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zamazan4ik at tut.by
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Consider the example:

unsigned long long sample2(unsigned long long m) {
    if (m >= 100) __builtin_unreachable();
    m *= 16;
    return m >> 3;
}

After the `if` statement we do know that the higher bits are set to 0. So
instead of generating the following assembly:

sample2(unsigned long long):
  mov rax, rdi
  sal rax, 4
  shr rax, 3
  ret

A more optimal assembly could be generated:

sample2(unsigned long long):
  lea rax, [rdi + rdi]
  ret


Godbolt playground: https://godbolt.org/z/1iSpTh

Same issues for GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91881

P.S.: that optimization is important for std::to_chars(..., double) like
functions, where a significant of a double is extracted into an unsigned long
long variable, so its upper bits are always zero.

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