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From: "svfuerst at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/51838] New: Inefficient add of 128 bit quantity represented as 64 bit tuple to 128 bit integer.
Date: Thu, 12 Jan 2012 19:29:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51838

             Bug #: 51838
           Summary: Inefficient add of 128 bit quantity represented as 64
                    bit tuple to 128 bit integer.
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: svfuerst@gmail.com


void foo(__uint128_t *x, unsigned long long y, unsigned long long z)
{
    *x += y + ((__uint128_t) z << 64);
}

Compiles into:

mov    %rdx,%r8
mov    %rsi,%rax
xor    %edx,%edx
add    (%rdi),%rax
mov    %rdi,%rcx
adc    0x8(%rdi),%rdx
xor    %esi,%esi
add    %rsi,%rax
adc    %r8,%rdx
mov    %rax,(%rcx)
mov    %rdx,0x8(%rcx)
retq 

The above can be optimized into:

add    %rsi, (%rdi)
adc    %rdx, 8(%rdi)
retq