[llvm-bugs] [Bug 49225] New: Missing optimization: SUBCARRY (SBB) not used

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49225

            Bug ID: 49225
           Summary: Missing optimization: SUBCARRY (SBB) not used
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: chfast at gmail.com
                CC: llvm-bugs at lists.llvm.org

This reports an issue where a sequence of additions with carry propagation are
nicely optimized to ADDCARRY nodes / ADC instruction. While the symmetric code
for subtraction is not optimized to SUBCARRY / SBB.

The reason may be that in the DAG combiner the combineCarryDiamond() and/or the
combineADDCARRYDiamond() is not properly specified for SUB case. Or
canonicalization causes the SUB case not being recognized (you can see that the
last SUB in sequence is converted to ADD).

https://godbolt.org/z/7cr1o9


using uint64_t = unsigned long;


bool uaddo(uint64_t al, uint64_t ah, uint64_t bl, uint64_t bh, uint64_t* ol,
uint64_t* oh) noexcept
{
    uint64_t l = al + bl;
    bool k0 = l < al;

    uint64_t t = ah + bh;
    bool k1 = t < ah;
    uint64_t h = t + k0;
    bool k2 = h < t;
    bool k = k1 || k2;

    *ol = l;
    *oh = h;
    return k;
}

bool usubo(uint64_t al, uint64_t ah, uint64_t bl, uint64_t bh, uint64_t* ol,
uint64_t* oh) noexcept
{
    uint64_t l = al - bl;
    bool k0 = l > al;

    uint64_t t = ah - bh;
    bool k1 = t > ah;
    uint64_t h = t - k0;
    bool k2 = h > t;
    bool k = k1 || k2;

    *ol = l;
    *oh = h;
    return k;
}


define dso_local zeroext i1 @_Z5uaddommmmPmS_(i64 %0, i64 %1, i64 %2, i64 %3,
i64* nocapture %4, i64* nocapture %5) local_unnamed_addr #0 {
  %7 = add i64 %2, %0
  %8 = icmp ult i64 %7, %0
  %9 = add i64 %3, %1
  %10 = icmp ult i64 %9, %1
  %11 = zext i1 %8 to i64
  %12 = add i64 %9, %11
  %13 = icmp ult i64 %12, %9
  %14 = or i1 %10, %13
  store i64 %7, i64* %4, align 8, !tbaa !2
  store i64 %12, i64* %5, align 8, !tbaa !2
  ret i1 %14
}

define dso_local zeroext i1 @_Z5usubommmmPmS_(i64 %0, i64 %1, i64 %2, i64 %3,
i64* nocapture %4, i64* nocapture %5) local_unnamed_addr #0 {
  %7 = sub i64 %0, %2
  %8 = icmp ult i64 %0, %2
  %9 = sub i64 %1, %3
  %10 = icmp ult i64 %1, %3
  %11 = sext i1 %8 to i64
  %12 = add i64 %9, %11
  %13 = icmp ugt i64 %12, %9
  %14 = or i1 %10, %13
  store i64 %7, i64* %4, align 8, !tbaa !2
  store i64 %12, i64* %5, align 8, !tbaa !2
  ret i1 %14
}

attributes #0 = { nofree norecurse nounwind uwtable willreturn writeonly
mustprogress "disable-tail-calls"="false" "frame-pointer"="none"
"less-precise-fpmad"="false" "min-legal-vector-width"="0"
"no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false" "no-trapping-math"="true"
"stack-protector-buffer-size"="8" "target-cpu"="x86-64"
"target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic"
"unsafe-fp-math"="false" "use-soft-float"="false" }



uaddo(unsigned long, unsigned long, unsigned long, unsigned long, unsigned
long*, unsigned long*):                       # @uaddo(unsigned long, unsigned
long, unsigned long, unsigned long, unsigned long*, unsigned long*)
        add     rdi, rdx
        adc     rsi, rcx
        setb    al
        mov     qword ptr [r8], rdi
        mov     qword ptr [r9], rsi
        ret
usubo(unsigned long, unsigned long, unsigned long, unsigned long, unsigned
long*, unsigned long*):                       # @usubo(unsigned long, unsigned
long, unsigned long, unsigned long, unsigned long*, unsigned long*)
        sub     rsi, rcx
        setb    cl
        sub     rdi, rdx
        mov     rdx, rsi
        sbb     rdx, 0
        cmp     rdx, rsi
        seta    al
        or      al, cl
        mov     qword ptr [r8], rdi
        mov     qword ptr [r9], rdx
        ret

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