[llvm-dev] proposal for optimization method (original) (raw)

Eli Friedman via llvm-dev llvm-dev at lists.llvm.org
Thu Feb 21 11:55:47 PST 2019

There’s a work-in-progress patch for the case r=0: https://reviews.llvm.org/D50222 . (Not sure what the current status of that patch is.)

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Craig Topper via llvm-dev Sent: Wednesday, February 20, 2019 9:27 PM To: or dv <dvoreader at gmail.com> Cc: llvm-dev <llvm-dev at lists.llvm.org> Subject: [EXT] Re: [llvm-dev] proposal for optimization method

It looks like gcc already does something like this. https://godbolt.org/z/JWljkL

~Craig

On Wed, Feb 20, 2019 at 8:45 PM or dv via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote: Hello everyone, I discovered a way to perform optimization on the following code (I gave an example that uses 32bit integer, but it works with any size.):

const uint32 d,r;//d is an odd number //d is the divisor, r is the remainder bool check_remainder(uint32 x) { return x%d==r; }

if we know d and r at compile time, and d is an odd integer, we can use modular multiplicative inverse to bypass the division operation. I wrote the following code to calculate the M.M.I of a number over 2^b. I give anyone the permission to use it (unlicensed).

uint64_t find_mod_mul_inverse(uint64_t x, uint64_t bits)

{

    if (bits > 64 || ((x&1)==0))

            return 0;// invalid parameters

    uint64_t mask;

    if (bits == 64)

            mask = -1;

    else

    {

            mask = 1;

            mask<<=bits;

            mask--;

    }

    x&=mask;

    uint64_t result=1, state=x, ctz=0;

    while(state!=1ULL)

    {

            ctz=__builtin_ctzll(state^1);

            result|=1ULL<<ctz;

            state+=x<<ctz;

            state&=mask;

    }

    return result;

}

now consider the following steps:

from the 2 constants (d and r) we create 3 constants (with the same bit length):

constants uint32 s,u,mmi;

mmi = find_mod_mul_inverse(d,32);

s = (r*mmi);

u = (UINT32_MAX-r)/d; // UINT32_MAX corresponds to pow(2,32)-1.

the idea behind these constants is the following formula:

mmi_of(d)*x=x/d+(x%d)*mmi_of(d)

now after we generated the constants, we will just emit the following code instead of the former:

bool check_remainder(uint32 x)

{

return ((x*mmi)-s)<=u;

}

Anyway, I looked at the file IntegerDivision.cpp, it seems to me that this new optimization is more effective then the optimization used there. However, I have no experience with compiler development, so I can just give you my idea. if further explanation is needed, just ask. I tested my method and it gives the correct results.

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