RFR 8249893: AARCH64: optimize the construction of the value from the bits of the other two

[Andrew Haley]

On 25/08/2020 18:30, Boris Ulasevich wrote:
> I believe masking with left shift and right shift is not common.
> Search though jdk repository does not give such patterns while
> there is a hundreds of mask+lshift expressions.


> I implemented a simple is_bitrange_zero() method for counting the
> bitranges of sub-expressions: power-of-two masks and left shift only.
> We can take into account more cases (careful testing is a main
> concern). But particularly about "r.a << 24 >>> 24" expression
> I think it is worse to think about canonicalization: "left shift + right
> shift" to "mask + left shift" (or may be the backwards).

I'm running your test program, and for example I get this, old on the
left, new on the right.

Compiled method (c2)   11832 1113             SubTest0::tst2 (184 bytes)

  : and       x11, x2, #0x1   ;*land                            :   and     x11, x2, #0x1
  : and       x10, x1, #0x1   ;*land                            :   and     x10, x1, #0x1
  : orr       x11, x11, x11, lsl #3                             :   bfi     x11, x2, #3, #1
  : orr       x10, x10, x10, lsl #3                             :   bfi     x10, x1, #3, #1
  : and       xmethod, x3, #0x1  ;*land                         :   and     xmethod, x3, #0x1
  : add       x10, x10, x11                                     :   bfi     xmethod, x3, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, x11
  : and       xmethod, x4, #0x1  ;*land                         :   and     x11, x4, #0x1
  : add       x10, x11, x10                                     :   bfi     x11, x4, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, xmethod
  : and       xmethod, x5, #0x1  ;*land                         :   and     xmethod, x5, #0x1
  : add       x10, x11, x10                                     :   bfi     xmethod, x5, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, x11
  : and       xmethod, x6, #0x1  ;*land                         :   and     x11, x6, #0x1
  : add       x10, x11, x10                                     :   bfi     x11, x6, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, xmethod
  : and       xmethod, x7, #0x1  ;*land                         :   and     xmethod, x7, #0x1
  : add       x10, x11, x10                                     :   bfi     xmethod, x7, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, x11
  : and       xmethod, x0, #0x1  ;*land                         :   add     x10, x10, xmethod
  : add       x10, x11, x10                                     :   ldr     x13, [sp,#32]
  : orr       x11, xmethod, xmethod, lsl #3                     :   and     x11, x0, #0x1
  : ldr       xmethod, [sp,#32]                                 :   and     xmethod, x13, #0x1
  : and       xmethod, xmethod, #0x1                            :   bfi     x11, x0, #3, #1
  : add       x10, x11, x10                                     :   bfi     xmethod, x13, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, x11
  : ldr       xmethod, [sp,#40]                                 :   ldr     x13, [sp,#40]
  : and       xmethod, xmethod, #0x1                            :   and     x11, x13, #0x1
  : add       x10, x11, x10                                     :   bfi     x11, x13, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, xmethod
  : ldr       xmethod, [sp,#48]                                 :   ldr     x13, [sp,#48]
  : and       xmethod, xmethod, #0x1                            :   and     xmethod, x13, #0x1
  : add       x10, x11, x10                                     :   bfi     xmethod, x13, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, x11
  : ldr       xmethod, [sp,#56]                                 :   ldr     x13, [sp,#56]
  : and       xmethod, xmethod, #0x1                            :   and     x11, x13, #0x1
  : add       x10, x11, x10                                     :   bfi     x11, x13, #3, #1
  : orr       x11, xmethod, xmethod, lsl #3                     :   add     x10, x10, xmethod
  : add       x0, x11, x10    ;*ladd                            :   add     x0, x10, x11

I've also tried a bunch of different test cases doing operations that
could match BFI instructions, and in only a few of them does it
happen. In almost all cases, then, this change does not help, *even
your own test case*.

I think that you've got something that is potentially useful, but it
needs some careful analysis to make sure it actually gets used.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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