ARM microoptimizations to megamorphic stub probing.

ARM microoptimizations to megamorphic stub probing.
Committed: https://code.google.com/p/v8/source/detail?r=10591

[Erik Corry, 2012-02-02 12:45:19]

Changes this:


  0x43210174  e308c202       movw ip, #33282
  0x43210178  e34fcfff       movt ip, #65535
  0x4321017c  e023300c       eor r3, r3, ip
  0x43210180  e301cffc       movw ip, #8188
  0x43210184  e003300c       and r3, r3, ip
  0x43210188  e3084188       movw r4, #33160
  0x4321018c  e340483c       movt r4, #2108
  0x43210190  e794c083       ldr ip, [r4, +r3, lsl #1]
  0x43210194  e152000c       cmp r2, ip
  0x43210198  1a00000b       bne +52 -> 0x432101cc
  0x4321019c  e2844004       add r4, r4, #4
  0x432101a0  e7945083       ldr r5, [r4, +r3, lsl #1]
  0x432101a4  e5955017       ldr r5, [r5, #+23]
  0x432101a8  e300c878       movw ip, #2168
  0x432101ac  e1c5500c       bic r5, r5, ip
  0x432101b0  e308c202       movw ip, #33282
  0x432101b4  e34fcfff       movt ip, #65535
  0x432101b8  e155000c       cmp r5, ip

to this


  0x2d611034  e2233b02       eor r3, r3, #2048
  0x2d611038  e7ea3153       ubfx r3, r3, #2, #11
  0x2d61103c  e3094370       movw r4, #37744
  0x2d611040  e340484d       movt r4, #2125
  0x2d611044  e794c183       ldr ip, [r4, +r3, lsl #3]
  0x2d611048  e152000c       cmp r2, ip
  0x2d61104c  1a000009       bne +44 -> 0x2d611078
  0x2d611050  e2844004       add r4, r4, #4
  0x2d611054  e7945183       ldr r5, [r4, +r3, lsl #3]
  0x2d611058  e595501b       ldr r5, [r5, #+27]
  0x2d61105c  e3c550f8       bic r5, r5, #248
  0x2d611060  e307c7fe       movw ip, #30718
  0x2d611064  e175000c       cmn r5, ip

[Michael Starzinger, 2012-02-02 13:14:19]

LGTM (if comment is addressed).

https://chromiumcodereview.appspot.com/9323002/diff/1/src/objects.h
File src/objects.h (right):

https://chromiumcodereview.appspot.com/9323002/diff/1/src/objects.h#newcode4376
src/objects.h:4376: // This constant should be encodable in an ARM instruction.
I think we need to have a static-assert at the usage site in stub-cache-arm.cc
where this is relevant. Otherwise that promise will be broken soon.