runtime: [Perf] Regressions in Integer Formatting

As a side note, while looking at the diff I noticed some other general improvements that could be made in the future:

sxtw    x3, w6	
lsl     x3, x3, #1	
add     x2, x2, x3

can become

add     x2, x2, w6, sxtw, #1

and

lsl     w3, w3, #1
sub     w3, w0, w3

can become

sub     w3, w0, w3, lsl 1

lastly

sub     x2, x2, #2
strh    w1, [x2]

can become

strh    w1, [x2, #-2]!

and probably want to move the constant out of the loop.

So I managed to finally generate a diff: https://www.diffchecker.com/Rr7cRI2V I suspect the slowdown comes from using mul Xd, Xn, Xm instead of umull Xd, Wn, Wm?

I think this is correct culprid, the loop has a long dependency chain off the value out of the mul/umull with little independent instructions so it’s bound by these two and 64-bit mul has twice the latency of umull.

Edit: ARM software optimization guides seem to indicate that mul and umull have identical latencies/throughput. umulh is more expensive, but it’s only used once and it should still be faster than a bunch of lsr/sub/add instructions…

64-bit mul has a latency of 4(3) and umull has 2(1) on Cortex-A76. Were you perhaps comparing the AArch32 instructions? Note that on AArch64 mul and umull aren’t real instructions but are architectural aliases for madd and umaddl respectively so you need to look at the latencies for those.

The mul seems to be unneeded though, both inputs are 32-bits so you should be able to just use umull there as before.

Test history implicate #52893. In particular the change was merged, reverted, and then re-merged, which matches the up-down-up seen here:

As for how to get diffs, it should be something like:

;; builds as you do above, in both "ref" and "diff" repos

jit-diff diff --base --base_root <path to ref repo> --core_root <path to release core_root in either repo> --altjit clrjit_win_arm64_x64.dll -t X --pmi

jit-diff diff --diff --diff_root <path to diff repo> --core_root <path to release core_root in either repo> --altjit clrjit_win_arm64_x64.dll -t X --pmi

jit-analyze --base X\base --diff X\diff