Double constants usage in a loop can be CSEed

[kunalspathak]

Doubles present in a loop are reloaded repeatedly. Instead they can be set just once out of loop and use it inside the loop.

private static double Process(double n)
{
  double res;
  res = 1;
  while (n > 0.0)
  {
    res *= n;
// User might not write such code, but here it is written merely to show that same constants are re-loaded in assembly code.
    n -= 1.0;
    n -= 2.0;
    n -= 1.0;
    n -= 2.0;
  }
  return res;
}

G_M15653_IG03:
        1E600A10          fmul    d16, d16, d0
        1E6E1011          fmov    d17, #1.0000
        1E713800          fsub    d0, d0, d17
        1E601011          fmov    d17, #2.0000
        1E713800          fsub    d0, d0, d17
        1E6E1011          fmov    d17, #1.0000
        1E713800          fsub    d0, d0, d17
        1E601011          fmov    d17, #2.0000
        1E713800          fsub    d0, d0, d17
        4F00E411          movi    v17.16b, #0x00
        1E712000          fcmp    d0, d17
        54FFFEAC          bgt     G_M15653_IG03

category:cq
theme:cse
skill-level:expert
cost:medium
impact:medium

[kunalspathak]

@CarolEidt @BruceForstall

[CarolEidt]

As I discussed offline with @kunalspathak , doing this without unduly pessimizing cases with high register pressure may also require rematerialization (#6264)

[EgorBo]

I guess this one is not arm specific as I see exactly the same picture on x86
(constants are not saved to registers before the loop)

[kunalspathak]

I guess this one is not arm specific as I see exactly the same picture on x86
(SIMD types aren't currently hoisted out of loops)

Agree. I will update the title/label.

[tannergooding]

For reference on the x64 side: https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AXEBDAzgWwB8ABAJgEYBYAKGIGYACMhgYQYG8aHuGAHKAJYA3bBhhNySBgBMIAV2AAbcQAUoEMDFy4AFLIXKGAOwCUNTtW76l42LgDcXBnYYBeBuUeWGAdwAWAoY6RgwAfAwADAB0EWbeFjzOWgwAVO5GXgD0mQwAqrgwUAz4AgDmfhjGEJU+gmIMuHJgfgyQ0jBoDMBylX6F4gKVAri+dWIh+P2KAJ4MGBANfhA+c36iDdiTrRBGuBjYRhgj2LBJcIoQ2O3SDAIheAX4SrNtMFFO3CFw7uQxXolfdykP4fYwMb4eEHeT7goFQ7gAXycxAA7EkHDQkdQEUA===

[EgorBo]

For reference on the x64 side: https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AXEBDAzgWwB8ABAJgEYBYAKGIGYACMhgYQYG8aHuGAHKAJYA3bBhhNySBgBMIAV2AAbcQAUoEMDFy4AFLIXKGAOwCUNTtW76l42LgDcXBnYYBeBuUeWGAdwAWAoY6RgwAfAwADAB0EWbeFjzOWgwAVO5GXgD0mQwAqrgwUAz4AgDmfhjGEJU+gmIMuHJgfgyQ0jBoDMBylX6F4gKVAri+dWIh+P2KAJ4MGBANfhA+c36iDdiTrRBGuBjYRhgj2LBJcIoQ2O3SDAIheAX4SrNtMFFO3CFw7uQxXolfdykP4fYwMb4eEHeT7goFQ7gAXycxAA7EkHDQkdQEUA===

Ah, by "hoisted out of loop" I meant to save 1 and 2 to some xmm registers before the loop.
Not sure it makes sense here tho.

gcc saves them: https://godbolt.org/z/gToudN