[llvm-bugs] [Bug 24545] New: [x86] Silly code generation for _addcarry_u32/_addcarry_u64

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=24545

            Bug ID: 24545
           Summary: [x86] Silly code generation for
                    _addcarry_u32/_addcarry_u64
           Product: libraries
           Version: 3.7
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: myriachan at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

x86 intrinsics _addcarry_u32 and _addcarry_u64 generate silly code.  For
example, the following function to get the result of a 64-bit addition (the XOR
is to the output clearer):

    u64 testcarry(u64 a, u64 b, u64 c, u64 d)
    {
        u64 result0, result1;
        _addcarry_u64(_addcarry_u64(0, a, c, &result0), b, d, &result1);
        return result0 ^ result1;
    }

This is the code generated with -O1, -O2 and -O3:

    xor    eax, eax
    add    al, -1
    adc    rdi, rdx
    mov    qword ptr [rsp - 8], rdi
    setb    al
    add    al, -1
    adc    rsi, rcx
    mov    qword ptr [rsp - 16], rsi
    xor    rsi, qword ptr [rsp - 8]
    mov    rax, rsi
    ret

The first silliness is that _addcarry_u64 does not optimize a compile-time
constant 0 being the first carry parameter.  Instead of "adc", it should just
use "add".

The second silliness is with the use of r8b to store the carry flag, then using
"add r8b, -1" to put the result back into carry.

The third silliness is that _addcarry_u64 is taking its pointer parameter too
literally; it shouldn't be storing values to memory at all.

Instead, the code should be something like this:

    add    rdx, rdi
    mov    rax, rdx
    adc    rcx, rsi
    xor    rax, rcx
    ret

Naturally, for something this simple, I'd use unsigned __int128, but this came
up in large number math.

Cross-filed with GCC (with different generated code since it's a different
compiler =^-^=):
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67317

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