Add support for rustc_boehm

[jacob-hughes]

This PR adds support for yksom to be used with rustc_boehm. If rustc_boehm is used to build yksom, the rustc_boehm feature flag can be passed to enable additional optimizations. E.g.:

cargo build --release --features "rustc_boehm"

I've identified two types in yksom which will benefit from the NoFinalize marker trait in rustc_boehm: Block; and String_. With that in place, we perform a lot better than ToT in the rebench suite:

----------------------------------------------------------------------------------------------------
  Benchmark     Executor                  Suite   Extra                 #Samples   Mean (ms)
----------------------------------------------------------------------------------------------------
  Bounce        baseline      micro       2      1                      10         109
  Bounce        rustc_boehm   micro       2      1                      10          29
  BubbleSort    baseline      micro       3      1                      10         103
  BubbleSort    rustc_boehm   micro       3      1                      10          31
  DeltaBlue     baseline      macro      50      1                      10         114
  DeltaBlue     rustc_boehm   macro      50      1                      10          24
  Dispatch      baseline      micro       2      1                      10          73
  Dispatch      rustc_boehm   micro       2      1                      10          21
  Fannkuch      baseline      micro       6      1                      10          52
  Fannkuch      rustc_boehm   micro       6      1                      10          17
  Fibonacci     baseline      micro       3      1                      10         237
  Fibonacci     rustc_boehm   micro       3      1                      10          52
  FieldLoop     baseline      micro       1      1                      10          53
  FieldLoop     rustc_boehm   micro       1      1                      10          25
  GraphSearch   baseline      macro       4      1                      10          29
  GraphSearch   rustc_boehm   macro       4      1                      10          11
  IntegerLoop   baseline      micro       2      1                      10         139
  IntegerLoop   rustc_boehm   micro       2      1                      10          34
  JsonSmall     baseline      macro       1      1                      10         105
  JsonSmall     rustc_boehm   macro       1      1                      10          42
  List          baseline      micro       2      1                      10         112
  List          rustc_boehm   micro       2      1                      10          31
  Loop          baseline      micro       5      1                      10         183
  Loop          rustc_boehm   micro       5      1                      10          47
  Mandelbrot    baseline      micro      30      1                      10         182
  Mandelbrot    rustc_boehm   micro      30      1                      10          58
  NBody         baseline      macro     500      1                      10          88
  NBody         rustc_boehm   macro     500      1                      10          41
  PageRank      baseline      macro      40      1                      10         110
  PageRank      rustc_boehm   macro      40      1                      10          35
  Permute       baseline      micro       3      1                      10          96
  Permute       rustc_boehm   micro       3      1                      10          29
  Queens        baseline      micro       2      1                      10          84
  Queens        rustc_boehm   micro       2      1                      10          26
  QuickSort     baseline      micro       1      1                      10          42
  QuickSort     rustc_boehm   micro       1      1                      10          12
  Recurse       baseline      micro       3      1                      10         107
  Recurse       rustc_boehm   micro       3      1                      10          27
  Richards      baseline      macro       1      1                      10        2183
  Richards      rustc_boehm   macro       1      1                      10         637
  Sieve         baseline      micro       4      1                      10         177
  Sieve         rustc_boehm   micro       4      1                      10          47
  Storage       baseline      micro       1      1                      10          57
  Storage       rustc_boehm   micro       1      1                      10          13
  Sum           baseline      micro       2      1                      10          74
  Sum           rustc_boehm   micro       2      1                      10          18
  Towers        baseline      micro       2      1                      10         123
  Towers        rustc_boehm   micro       2      1                      10          31
  TreeSort      baseline      micro       1      1                      10         135
  TreeSort      rustc_boehm   micro       1      1                      10          24
  WhileLoop     baseline      micro      10      1                      10         173
  WhileLoop     rustc_boehm   micro      10      1                      10          45

[ltratt]

Out of curiosity, what other types did you try marking as NoFinalize?

bors r+

[jacob-hughes]

Out of curiosity, what other types did you try marking as NoFinalize?

Those were the only ones that I could identify as valid candidates (other, more obvious types were already covered by needs_drop)

[ltratt]

bors ping

[bors[bot]]

pong

[ltratt]

Hmm, I'm not sure why bors didn't run earlier. Let's try again...

bors r+

[bors[bot]]

Build succeeded:

• buildbot/buildbot-build-script