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Currently, for stack objects, the compiler emits metadata that
includes the offset and type descriptor for each object. The type
descriptor symbol has many fields, and it references many other
symbols, e.g. field/element types, equality functions, names.
Observe that what we actually need at runtime is only the GC
metadata that are needed to scan the object, and the GC metadata
are "leaf" symbols (which doesn't reference other symbols). Emit
only the GC data instead. This avoids bringing live the type
descriptor as well as things referenced by it (if it is not
otherwise live).
This reduces binary sizes:
old new
hello (println) 1187776 1133856 (-4.5%)
hello (fmt) 1902448 1844416 (-3.1%)
cmd/compile 22670432 22438576 (-1.0%)
cmd/link 6346272 6225408 (-1.9%)
No significant change in compiler speed.
name old time/op new time/op delta
Template 184ms ± 2% 186ms ± 5% ~ (p=0.905 n=9+10)
Unicode 78.4ms ± 5% 76.3ms ± 3% -2.60% (p=0.009 n=10+10)
GoTypes 1.09s ± 1% 1.08s ± 1% -0.73% (p=0.027 n=10+8)
Compiler 85.6ms ± 3% 84.6ms ± 4% ~ (p=0.143 n=10+10)
SSA 7.23s ± 1% 7.25s ± 1% ~ (p=0.780 n=10+9)
Flate 116ms ± 5% 115ms ± 6% ~ (p=0.912 n=10+10)
GoParser 201ms ± 4% 195ms ± 1% ~ (p=0.089 n=10+10)
Reflect 455ms ± 1% 458ms ± 2% ~ (p=0.050 n=9+9)
Tar 155ms ± 2% 155ms ± 3% ~ (p=0.436 n=10+10)
XML 202ms ± 2% 200ms ± 2% ~ (p=0.053 n=10+9)
Change-Id: I33a7f383d79afba1a482cac6da0cf5b7de9c0ec4
Reviewed-on: https://go-review.googlesource.com/c/go/+/313514
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
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Right now we just prevent such types from being on the heap. This CL
makes it so they cannot appear on the stack either. The distinction
between heap and stack is pretty vague at the language level (e.g. it
is affected by -N), and we don't need the flexibility anyway.
Once go:notinheap types cannot be in either place, we don't need to
consider pointers to such types to be pointers, at least according to
the garbage collector and stack copying. (This is the big win of this
CL, in my opinion.)
The distinction between HasPointers and HasHeapPointer no longer
exists. There is only HasPointers.
This CL is cleanup before possible use of go:notinheap to fix #40954.
Update #13386
Change-Id: Ibd895aadf001c0385078a6d4809c3f374991231a
Reviewed-on: https://go-review.googlesource.com/c/go/+/249917
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
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This adds support for scanning the stack when a goroutine is stopped
at an async safe point. This is not yet lit up because asyncPreempt is
not yet injected, but prepares us for that.
This works by conservatively scanning the registers dumped in the
frame of asyncPreempt and its parent frame, which was stopped at an
asynchronous safe point.
Conservative scanning works by only marking words that are pointers to
valid, allocated heap objects. One complication is pointers to stack
objects. In this case, we can't determine if the stack object is still
"allocated" or if it was freed by an earlier GC. Hence, we need to
propagate the conservative-ness of scanning stack objects: if all
pointers found to a stack object were found via conservative scanning,
then the stack object itself needs to be scanned conservatively, since
its pointers may point to dead objects.
For #10958, #24543.
Change-Id: I7ff84b058c37cde3de8a982da07002eaba126fd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/201761
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
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Change-Id: I904b8655f21743189814bccf24073b6fbb9fc56d
GitHub-Last-Rev: b032c14394c949f9ad7b18d019a3979d38d4e1fb
GitHub-Pull-Request: golang/go#29997
Reviewed-on: https://go-review.googlesource.com/c/160421
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
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Rework how the compiler+runtime handles stack-allocated variables
whose address is taken.
Direct references to such variables work as before. References through
pointers, however, use a new mechanism. The new mechanism is more
precise than the old "ambiguously live" mechanism. It computes liveness
at runtime based on the actual references among objects on the stack.
Each function records all of its address-taken objects in a FUNCDATA.
These are called "stack objects". The runtime then uses that
information while scanning a stack to find all of the stack objects on
a stack. It then does a mark phase on the stack objects, using all the
pointers found on the stack (and ancillary structures, like defer
records) as the root set. Only stack objects which are found to be
live during this mark phase will be scanned and thus retain any heap
objects they point to.
A subsequent CL will remove all the "ambiguously live" logic from
the compiler, so that the stack object tracing will be required.
For this CL, the stack tracing is all redundant with the current
ambiguously live logic.
Update #22350
Change-Id: Ide19f1f71a5b6ec8c4d54f8f66f0e9a98344772f
Reviewed-on: https://go-review.googlesource.com/c/134155
Reviewed-by: Austin Clements <austin@google.com>
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