[llvm-dev] Unexplained absolute addressing mode generated on amd64/FreeBSD. Bug?

[Robert Clausecker via llvm-dev]

With some help from the IRC, I managed to use creduce to cut down the
test case.  The result is this:

    double a;
    b;
    c() {
        while (b)
            if (a)
                d();
    }

This produces code like this (slightly cleaned up):

    .LCPI0_0:
            .quad   0
    a:      .quad   0
    b:      .long   0
    c:      pushq   %rbp
            movq    %rsp, %rbp
    .LBB0_1:
            movl    b(%rip), %eax
    .LBB0_2:
            testl   %eax, %eax
            je      .LBB0_5
            movsd   a(%rip), %xmm0
            ucomisd .LCPI0_0, %xmm0            ; ???
            jne     .LBB0_4
            jnp     .LBB0_2
    .LBB0_4:
            xorl    %eax, %eax
            callq   d
            jmp     .LBB0_1
    .LBB0_5:
            popq    %rbp
            retq

Why is an absolute addressing mode instead of a rip-relative addressing
mode chosen for the memory operand to ucomisd?  And why is zero loaded
from memory at all?  I might not be an expert, but

            xorsd   %xmm0, %xmm0
            ucomisd a(%rip), %xmm0

looks a bit better than the movsd/ucomisd pair clang comes up with.

Yours,
Robert Clausecker

Am Mon, Feb 22, 2021 at 06:33:17PM +0100 schrieb Robert Clausecker via llvm-dev:
> Good evening!
> 
> I'm experiencing weird code generation on amd64 FreeBSD with the
> system's clang 10.0.1.  specifically, an absolute addressing mode is
> unexpectedly chosen for ucomisd with a memory constant where I would
> normally expect a rip-relative addressing mode.  The behaviour has
> proven to be hard to reproduce.  Any attempt to simplify the code leads
> to perfectly fine code.  To reproduce, download my project:
> 
>     http://fuz.su/~fuz/dobutsu/dobutsu-2.tar.xz
> 
> and compile dobutsu.c with -O3 on amd64/FreeBSD (Linux should work,
> too).  The output has a bunch of instructions like this:
> 
>     ucomisd .LCPI0_0, %xmm1
> 
> with an absolute addressing mode where I would expect a relative
> addressing mode like
> 
>     ucomisd .LCPI0_0(%rip), %xmm1
> 
> I wonder what the reason for this could be.  I can reproduce this
> error with clang 11.0.1 from FreeBSD but haven't been able to test
> 12.0.  Will continue to try and reduce the example.
> 
> Yours,
> Robert Clausecker
> 
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