[Bug target/82618] Inefficient double-word subtration on x86_64

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82618

--- Comment #3 from Uroš Bizjak <ubizjak at gmail dot com> ---
Created attachment 42402
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42402&action=edit
Prototype patch

Attached patch compiles the test to:

        movq    %rsi, %r10
        cmpq    %rdx, %rdi
        sbbq    %rcx, %r10
        movq    %r10, %rax