[llvm-bugs] [Bug 41647] New: does not eliminate a branch on shifts

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41647

            Bug ID: 41647
           Summary: does not eliminate a branch on shifts
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: slandden at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

When creating a mask, with unsigned n (I'm doing this in Zig because underflow
is undefined in Zig)

    if (n == 0) {
        return 0;
    } else {
        return u64(math.maxInt(u64)) >> @intCast(u6, ((64 - @intCast(u7, n))));
    }

mask:
        test    dil, dil
        je      .LBB0_1
        neg     dil
        mov     rax, -1
        shrx    rax, rax, rdi
        ret
.LBB0_1:
        xor     eax, eax
        ret

But the branch is unneeded.

https://www.felixcloutier.com/x86/sarx:shlx:shrx

If the value specified in the first source operand exceeds OperandSize -1, the
COUNT value is masked.

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