The write barrier shadow heap was very useful for
developing the write barriers initially, but it's no longer used,
clunky, and dragging the rest of the implementation down.

The gccheckmark mode will find bugs due to missed barriers
when they result in missed marks; wbshadow mode found the
missed barriers more aggressively, but it required an entire
separate copy of the heap. The gccheckmark mode requires
no extra memory, making it more useful in practice.

Compared to previous CL:
name                   old mean              new mean              delta
BinaryTree17            5.91s × (0.96,1.06)   5.72s × (0.97,1.03)  -3.12% (p=0.000)
Fannkuch11              4.32s × (1.00,1.00)   4.36s × (1.00,1.00)  +0.91% (p=0.000)
FmtFprintfEmpty        89.0ns × (0.93,1.10)  86.6ns × (0.96,1.11)    ~    (p=0.077)
FmtFprintfString        298ns × (0.98,1.06)   283ns × (0.99,1.04)  -4.90% (p=0.000)
FmtFprintfInt           286ns × (0.98,1.03)   283ns × (0.98,1.04)  -1.09% (p=0.032)
FmtFprintfIntInt        498ns × (0.97,1.06)   480ns × (0.99,1.02)  -3.65% (p=0.000)
FmtFprintfPrefixedInt   408ns × (0.98,1.02)   396ns × (0.99,1.01)  -3.00% (p=0.000)
FmtFprintfFloat         587ns × (0.98,1.01)   562ns × (0.99,1.01)  -4.34% (p=0.000)
FmtManyArgs            1.94µs × (0.99,1.02)  1.89µs × (0.99,1.01)  -2.85% (p=0.000)
GobDecode              15.8ms × (0.98,1.03)  15.7ms × (0.99,1.02)    ~    (p=0.251)
GobEncode              12.0ms × (0.96,1.09)  11.8ms × (0.98,1.03)  -1.87% (p=0.024)
Gzip                    648ms × (0.99,1.01)   647ms × (0.99,1.01)    ~    (p=0.688)
Gunzip                  143ms × (1.00,1.01)   143ms × (1.00,1.01)    ~    (p=0.203)
HTTPClientServer       90.3µs × (0.98,1.01)  89.1µs × (0.99,1.02)  -1.30% (p=0.000)
JSONEncode             31.6ms × (0.99,1.01)  31.7ms × (0.98,1.02)    ~    (p=0.219)
JSONDecode              107ms × (1.00,1.01)   111ms × (0.99,1.01)  +3.58% (p=0.000)
Mandelbrot200          6.03ms × (1.00,1.01)  6.01ms × (1.00,1.00)    ~    (p=0.077)
GoParse                6.53ms × (0.99,1.03)  6.54ms × (0.99,1.02)    ~    (p=0.585)
RegexpMatchEasy0_32     161ns × (1.00,1.01)   161ns × (0.98,1.05)    ~    (p=0.948)
RegexpMatchEasy0_1K     541ns × (0.99,1.01)   559ns × (0.98,1.01)  +3.32% (p=0.000)
RegexpMatchEasy1_32     138ns × (1.00,1.00)   137ns × (0.99,1.01)  -0.55% (p=0.001)
RegexpMatchEasy1_1K     887ns × (0.99,1.01)   878ns × (0.99,1.01)  -0.98% (p=0.000)
RegexpMatchMedium_32    253ns × (0.99,1.01)   252ns × (0.99,1.01)  -0.39% (p=0.001)
RegexpMatchMedium_1K   72.8µs × (1.00,1.00)  72.7µs × (1.00,1.00)    ~    (p=0.485)
RegexpMatchHard_32     3.85µs × (1.00,1.01)  3.85µs × (1.00,1.01)    ~    (p=0.283)
RegexpMatchHard_1K      117µs × (1.00,1.01)   117µs × (1.00,1.00)    ~    (p=0.175)
Revcomp                 922ms × (0.97,1.08)   903ms × (0.98,1.05)  -2.15% (p=0.021)
Template                126ms × (0.99,1.01)   126ms × (0.99,1.01)    ~    (p=0.943)
TimeParse               628ns × (0.99,1.01)   634ns × (0.99,1.01)  +0.92% (p=0.000)
TimeFormat              668ns × (0.99,1.01)   698ns × (0.98,1.03)  +4.53% (p=0.000)

It's nice that the microbenchmarks are the ones helped the most,
because those were the ones hurt the most by the conversion from
4-bit to 2-bit heap bitmaps. This CL brings the overall effect of that
process to (compared to CL 9706 patch set 1):

name                   old mean              new mean              delta
BinaryTree17            5.87s × (0.94,1.09)   5.72s × (0.97,1.03)  -2.57% (p=0.011)
Fannkuch11              4.32s × (1.00,1.00)   4.36s × (1.00,1.00)  +0.87% (p=0.000)
FmtFprintfEmpty        89.1ns × (0.95,1.16)  86.6ns × (0.96,1.11)    ~    (p=0.090)
FmtFprintfString        283ns × (0.98,1.02)   283ns × (0.99,1.04)    ~    (p=0.681)
FmtFprintfInt           284ns × (0.98,1.04)   283ns × (0.98,1.04)    ~    (p=0.620)
FmtFprintfIntInt        486ns × (0.98,1.03)   480ns × (0.99,1.02)  -1.27% (p=0.002)
FmtFprintfPrefixedInt   400ns × (0.99,1.02)   396ns × (0.99,1.01)  -0.84% (p=0.001)
FmtFprintfFloat         566ns × (0.99,1.01)   562ns × (0.99,1.01)  -0.80% (p=0.000)
FmtManyArgs            1.91µs × (0.99,1.02)  1.89µs × (0.99,1.01)  -1.10% (p=0.000)
GobDecode              15.5ms × (0.98,1.05)  15.7ms × (0.99,1.02)  +1.55% (p=0.005)
GobEncode              11.9ms × (0.97,1.03)  11.8ms × (0.98,1.03)  -0.97% (p=0.048)
Gzip                    648ms × (0.99,1.01)   647ms × (0.99,1.01)    ~    (p=0.627)
Gunzip                  143ms × (1.00,1.00)   143ms × (1.00,1.01)    ~    (p=0.482)
HTTPClientServer       89.2µs × (0.99,1.02)  89.1µs × (0.99,1.02)    ~    (p=0.740)
JSONEncode             32.3ms × (0.97,1.06)  31.7ms × (0.98,1.02)  -1.95% (p=0.002)
JSONDecode              106ms × (0.99,1.01)   111ms × (0.99,1.01)  +4.22% (p=0.000)
Mandelbrot200          6.02ms × (1.00,1.00)  6.01ms × (1.00,1.00)    ~    (p=0.417)
GoParse                6.57ms × (0.97,1.06)  6.54ms × (0.99,1.02)    ~    (p=0.404)
RegexpMatchEasy0_32     162ns × (1.00,1.00)   161ns × (0.98,1.05)    ~    (p=0.088)
RegexpMatchEasy0_1K     561ns × (0.99,1.02)   559ns × (0.98,1.01)  -0.47% (p=0.034)
RegexpMatchEasy1_32     145ns × (0.95,1.04)   137ns × (0.99,1.01)  -5.56% (p=0.000)
RegexpMatchEasy1_1K     864ns × (0.99,1.04)   878ns × (0.99,1.01)  +1.57% (p=0.000)
RegexpMatchMedium_32    255ns × (0.99,1.04)   252ns × (0.99,1.01)  -1.43% (p=0.001)
RegexpMatchMedium_1K   73.9µs × (0.98,1.04)  72.7µs × (1.00,1.00)  -1.55% (p=0.004)
RegexpMatchHard_32     3.92µs × (0.98,1.04)  3.85µs × (1.00,1.01)  -1.80% (p=0.003)
RegexpMatchHard_1K      120µs × (0.98,1.04)   117µs × (1.00,1.00)  -2.13% (p=0.001)
Revcomp                 936ms × (0.95,1.08)   903ms × (0.98,1.05)  -3.58% (p=0.002)
Template                130ms × (0.98,1.04)   126ms × (0.99,1.01)  -2.98% (p=0.000)
TimeParse               638ns × (0.98,1.05)   634ns × (0.99,1.01)    ~    (p=0.198)
TimeFormat              674ns × (0.99,1.01)   698ns × (0.98,1.03)  +3.69% (p=0.000)

Change-Id: Ia0e9b50b1d75a3c0c7556184cd966305574fe07c
Reviewed-on: https://go-review.googlesource.com/9706Reviewed-by: Rick Hudson <rlh@golang.org>