ARM64: Optimize a % b operation

[kunalspathak]

Optimize a % b operation for ARM64 for following scenarios:

a is unsigned int, b is power of 2.

static uint PerformMod_1(uint i) {
 return i % 8;
}

Today we generate something like this:

 53037C01 lsr w1, w0, #3
 531D7021 lsl w1, w1, #3
 4B010000 sub w0, w0, w1

We can generate:

 and w2, w1, 3

a is signed int, b is power of 2.

static int PerformMod_2(int i) {
 return i % 16;
}

Today we generate:

G_M58511_IG02:
 131F7C01 asr w1, w0, #31
 12000C21 and w1, w1, #15
 0B000021 add w1, w1, w0
 13047C21 asr w1, w1, #4
 531C6C21 lsl w1, w1, #4
 4B010000 sub w0, w0, w1

We can generate:

 negs x1, x0
 and x0, x0, #(n - 1)
 and x1, x1, #(n - 1)
 csneg x0, x0, x1, mi

a is an int, b is a variable.

static uint PerformMod_3(int i, int j) {
 return i % j;
}

Today we generate:

 1AC10802 sdiv w2, w0, w1 # or udiv if a is unsigned
 1B017C41 mul w1, w2, w1
 4B010000 sub w0, w0, w1

We can generate using msub:

 sdiv w3, w2, w1
 msub w3, w3, w1, w2

Reference: https://godbolt.org/z/yxH8jZ
Reference: https://patchwork.kernel.org/patch/11126001/

category:cq
theme:optimization
skill-level:intermediate
cost:medium

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[kunalspathak]

@BruceForstall

[EgorBo]

Jit currently always gives up on % on Arm and always converts it to a - (a / b) * b in the morph phase, thus existing %(GT_MOD, GT_UMOD) optimizations in both morph and lowering don't work for it (happen later).

[kunalspathak]

Yes, I have already spoke to @sandreenko about it and reverted some of the changes he did in dotnet/coreclr#18206. With that it handles the 1st case (where a is unsigned and b is power of 2). I need to handle other 2 cases yet.