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A comment on Faster remainders when the divisor is a constant: beating compilers & libdivide. Parent comment

Re:Faster remainders when the divisor is a constant: beating... by acqq on Saturday February 09, 2019 @02:40AM (#19121217)
When input unsigned but before if < 99, output unsigned int:

clang 7

        mov     ecx, edi
        mov     eax, 3435973837
        imul    rax, rcx
        shr     rax, 35
gcc 8.2

        mov     eax, edi
        mov     edx, -858993459
        mul     edx
        mov     eax, edx
        shr     eax, 3
msvc 19

        mov     eax, -858993459  ; cccccccdH
        mul     edx
        shr     edx, 3
        mov     eax, edx
icc 19

        mov       eax, 1717986919                               
        imul      edx                                           
        sar       edx, 2                          
        mov       eax, edx
The same, but when input unsigned char:

clang 7

        movzx   eax, dil
        imul    eax, eax, 205
        shr     eax, 11
gcc 8.2

        movzx   edi, dil
        lea     eax, [rdi+rdi*4]
        lea     eax, [rdi+rax*8]
        lea     eax, [rax+rax*4]
        shr     ax, 11
        movzx   eax, al
msvc 19

        movzx   ecx, al
        mov     eax, -858993459  ; cccccccdH
        mul     ecx
        shr     edx, 3
        mov     eax, edx
icc 19

        mov       eax, 1717986919
        imul      edi            
        sar       edx, 2        
        mov       eax, edx
clang and gcc with -O3, msvc -O2
[ Parent | on HN ]
Re:Faster remainders when the divisor is a constant: beating... by saagarjha on Saturday February 09, 2019 @04:13AM (#19121461)
This was exactly what I was about to mention; I can reproduce your results in Clang but it seems GCC doesn't want to optimize this? Here's what Godbolt gives me: https://godbolt.org/z/aFTdur
[ Parent | on HN ]
Re:Faster remainders when the divisor is a constant: beating... by acqq on Saturday February 09, 2019 @10:24AM (#19122696)
Just write a function “mydiv10” which returns if x < 99 { x /10 and 0 otherwise and try again.
[ Parent | on HN ]
Re:Faster remainders when the divisor is a constant: beating... by acqq on Sunday February 10, 2019 @04:39PM (#19130218)
Like this:

https://godbolt.org/z/bS2yzB

But also note that when the input is unsigned short we get:

https://godbolt.org/z/sbDbMU

        movzx   eax, di
        imul    eax, eax, 52429
        shr     eax, 19
[ Parent | on HN ]