lld/test/MachO/arm64-thunk-starvation.s

   1 # REQUIRES: aarch64
   2 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t.o
   3 # RUN: %lld -arch arm64 -lSystem -o %t.out %t.o
   4
   5 ## Regression test for PR51578.
   6
   7 .subsections_via_symbols
   8
   9 .globl _f1, _f2, _f3, _f4, _f5, _f6
  10 .p2align 2
  11 _f1: b _fn1
  12 _f2: b _fn2
  13 _f3: b _fn3
  14 _f4: b _fn4
  15 _f5: b _fn5
  16 _f6: b _fn6
  17 ## 6 * 4 = 24 bytes for branches
  18 ## Currently leaves 12 bytes for one thunk, so 36 bytes.
  19 ## Uses < instead of <=, so 40 bytes.
  20
  21 .global _spacer1, _spacer2
  22 ## 0x8000000 is 128 MiB, one more than the forward branch limit,
  23 ## distributed over two functions since our thunk insertion algorithm
  24 ## can't deal with a single function that's 128 MiB.
  25 ## We leave just enough room so that the old thunking algorithm finalized
  26 ## both spacers when processing _f1 (24 bytes for the 4 bytes code for each
  27 ## of the 6 _f functions, 12 bytes for one thunk, 4 bytes because the forward
  28 ## branch range is 128 Mib - 4 bytes, and another 4 bytes because the algorithm
  29 ## uses `<` instead of `<=`, for a total of 44 bytes slop.) Of the slop, 20
  30 ## bytes are actually room for thunks.
  31 ## _fn1-_fn6 aren't finalized because then there wouldn't be room for a thunk.
  32 ## But when a thunk is inserted to jump from _f1 to _fn1, that needs 12 bytes
  33 ## but _f2 is only 4 bytes later, so after _f1 there are only
  34 ## 20-(12-4) = 12 bytes left, after _f2 only 12-(12-4) 4 bytes, and after
  35 ## _f3 there's no more room for thunks and we can't make progress.
  36 ## The fix is to leave room for many more thunks.
  37 ## The same construction as this test case can defeat that too with enough
  38 ## consecutive jumps, but in practice there aren't hundreds of consecutive
  39 ## jump instructions.
  40
  41 _spacer1:
  42 .space 0x4000000
  43 _spacer2:
  44 .space 0x4000000 - 44
  45
  46 .globl _fn1, _fn2, _fn3, _fn4, _fn5, _fn6
  47 .p2align 2
  48 _fn1: ret
  49 _fn2: ret
  50 _fn3: ret
  51 _fn4: ret
  52 _fn5: ret
  53 _fn6: ret
  54
  55 .globl _main
  56 _main:
  57   ret