newstackcall creates a new stack segment, and we want to
be able to throw away all that code.
LGTM=khr
R=khr, iant
CC=dvyukov, golang-codereviews, r
https://golang.org/cl/139270043
I did this just to clean things up, but it will be important
when we drop the pkg directory later.
LGTM=bradfitz
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/132600043
Behavior before this CL:
1. If onM is called on a g0 stack, it just calls the given function.
2. If onM is called on a gsignal stack, it calls badonm.
3. If onM is called on a curg stack, it switches to the g0 stack
and then calls the function.
In cases 1 and 2, if the program then crashes (and badonm always does),
we want to see what called onM, but the traceback stops at onM.
In case 3, the traceback must stop at onM, because the g0
stack we are renting really does stop at onM.
The current code stops the traceback at onM to handle 3,
at the cost of making 1 and 2 crash with incomplete traces.
Change traceback to scan past onM but in case 3 make it look
like on the rented g0 stack, onM was called from mstart.
The traceback already knows that mstart is a top-of-stack function.
Alternate fix at CL 132610043 but I think this one is cleaner.
This CL makes 3 the exception, while that CL makes 1 and 2 the exception.
Submitting TBR to try to get better stack traces out of the
freebsd/amd64 builder, but happy to make changes in a
followup CL.
TBR=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/133620043
The common code is converted, epoll and kqueue are converted.
Windows and solaris are still C.
LGTM=rsc
R=golang-codereviews, rsc, dave
CC=golang-codereviews, iant, khr, rsc
https://golang.org/cl/132910043
The implementation and use patterns of onM assume
that they run on either the m->curg or m->g0 stack.
Calling onM from m->gsignal has two problems:
(1) When not on g0, onM switches to g0 and then "back" to curg.
If we didn't start at curg, bad things happen.
(2) The use of scalararg/ptrarg to pass C arguments and results
assumes that there is only one onM call at a time.
If a gsignal starts running, it may have interrupted the
setup/teardown of the args for an onM on the curg or g0 stack.
Using scalararg/ptrarg itself would smash those.
We can fix (1) by remembering what g was running before the switch.
We can fix (2) by requiring that uses of onM that might happen
on a signal handling stack must save the old scalararg/ptrarg
and restore them after the call, instead of zeroing them.
The only sane way to do this is to introduce a separate
onM_signalsafe that omits the signal check, and then if you
see a call to onM_signalsafe you know the surrounding code
must preserve the old scalararg/ptrarg values.
(The implementation would be that onM_signalsafe just calls
fn if on the signal stack or else jumps to onM. It's not necessary
to have two whole copies of the function.)
(2) is not a problem if the caller and callee are both Go and
a closure is used instead of the scalararg/ptrarg slots.
For now, I think we can avoid calling onM from code executing
on gsignal stacks, so just reject it.
In the long term, (2) goes away (as do the scalararg/ptrarg slots)
once everything is in Go, and at that point fixing (1) would be
trivial and maybe worth doing just for regularity.
LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, khr, r
https://golang.org/cl/135400043
Instead of making asmcgocall call asmcgocall_errno,
make both load args into registers and call a shared
assembly function.
On amd64, this costs 1 word in the asmcgocall_errno path
but saves 3 words in the asmcgocall path, and the latter
is what happens on critical nosplit paths on Windows.
On arm, this fixes build failures: asmcgocall was writing
the arguments for asmcgocall_errno into the wrong
place on the stack. Passing them in registers avoids the
decision entirely.
On 386, this isn't really needed, since the nosplit paths
have twice as many words to work with, but do it for consistency.
Update #8635
Fixes arm build (except GOARM=5).
TBR=iant
CC=golang-codereviews
https://golang.org/cl/134390043
This gives them correct types in Go and also makes it
possible to use them to run Go code on an m stack.
LGTM=iant
R=golang-codereviews, dave, iant
CC=dvyukov, golang-codereviews, khr, r
https://golang.org/cl/137970044
They were in proc.c mainly because there was no portable
traceback source file. As part of converting them to Go,
move to traceback.go.
In order to get access to the PC of _rt0_go,
rename to runtime.rt0_go.
LGTM=r
R=golang-codereviews, r
CC=dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/139110043
C and Go calling conventions are now compatible, so we
don't need two versions of this function.
LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/139080043
Actually it mostly deletes code -- alg.print and alg.copy go away.
There was only one usage of alg.print for debug purposes.
Alg.copy is used in chan.goc, but Keith replaces them with
memcopy during conversion, so alg.copy is not needed as well.
Converting them would be significant amount of work
for no visible benefit.
LGTM=crawshaw, rsc, khr
R=golang-codereviews, crawshaw, khr
CC=golang-codereviews, rsc
https://golang.org/cl/139930044
uintptr or uint64 in the runtime C were turning into uint in the Go,
bool was turning into uint8, and so on. Fix that.
Also delete Go wrappers for C functions.
The C functions can be called directly now
(but still eventually need to be converted to Go).
LGTM=bradfitz, minux, iant
R=golang-codereviews, bradfitz, iant, minux
CC=golang-codereviews, khr, r
https://golang.org/cl/138740043
To date, the C compilers and Go compilers differed only in how
values were returned from functions. This made it difficult to call
Go from C or C from Go if return values were involved. It also made
assembly called from Go and assembly called from C different.
This CL changes the C compiler to use the Go conventions, passing
results on the stack, after the arguments.
[Exception: this does not apply to C ... functions, because you can't
know where on the stack the arguments end.]
By doing this, the CL makes it possible to rewrite C functions into Go
one at a time, without worrying about which languages call that
function or which languages it calls.
This CL also updates all the assembly files in package runtime to use
the new conventions. Argument references of the form 40(SP) have
been rewritten to the form name+10(FP) instead, and there are now
Go func prototypes for every assembly function called from C or Go.
This means that 'go vet runtime' checks effectively every assembly
function, and go vet's output was used to automate the bulk of the
conversion.
Some functions, like seek and nsec on Plan 9, needed to be rewritten.
Many assembly routines called from C were reading arguments
incorrectly, using MOVL instead of MOVQ or vice versa, especially on
the less used systems like openbsd.
These were found by go vet and have been corrected too.
If we're lucky, this may reduce flakiness on those systems.
Tested on:
darwin/386
darwin/amd64
linux/arm
linux/386
linux/amd64
If this breaks another system, the bug is almost certainly in the
sys_$GOOS_$GOARCH.s file, since the rest of the CL is tested
by the combination of the above systems.
LGTM=dvyukov, iant
R=golang-codereviews, 0intro, dave, alex.brainman, dvyukov, iant
CC=golang-codereviews, josharian, r
https://golang.org/cl/135830043
These are required for chans, semaphores, timers, etc.
LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/123640043
Half the code in the garbage collector accesses the bitmap
as an array of bytes instead of as an array of uintptrs.
This is tricky to do correctly in a portable fashion,
it breaks on big-endian systems.
Make the bitmap a byte array.
Simplifies markallocated, scanblock and span sweep along the way,
as we don't need to recalculate bitmap position for each word.
LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/125250043
Create proper closures so hash functions can be called
directly from Go. Rearrange calling convention so return
value is directly accessible.
LGTM=dvyukov
R=golang-codereviews, dvyukov, dave, khr
CC=golang-codereviews
https://golang.org/cl/119360043
The DISPATCH and CALLFN macro definitions depend on an inconsistency
between the internal cpp mini-implementation and the language proper in
whether center-dot is an identifier character. The macro depends on it not
being an identifier character, but the resulting code depends on it being one.
Remove the dependence on the inconsistency by placing the center-dot into
the macro invocation rather that the body.
No semantic change. This is just renaming macro arguments.
LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/119320043
This change introduces gomallocgc, a Go clone of mallocgc.
Only a few uses have been moved over, so there are still
lots of uses from C. Many of these C uses will be moved
over to Go (e.g. in slice.goc), but probably not all.
What should remain of C's mallocgc is an open question.
LGTM=rsc, dvyukov
R=rsc, khr, dave, bradfitz, dvyukov
CC=golang-codereviews
https://golang.org/cl/108840046
The runtime has historically held two dedicated values g (current goroutine)
and m (current thread) in 'extern register' slots (TLS on x86, real registers
backed by TLS on ARM).
This CL removes the extern register m; code now uses g->m.
On ARM, this frees up the register that formerly held m (R9).
This is important for NaCl, because NaCl ARM code cannot use R9 at all.
The Go 1 macrobenchmarks (those with per-op times >= 10 µs) are unaffected:
BenchmarkBinaryTree17 5491374955 5471024381 -0.37%
BenchmarkFannkuch11 4357101311 4275174828 -1.88%
BenchmarkGobDecode 11029957 11364184 +3.03%
BenchmarkGobEncode 6852205 6784822 -0.98%
BenchmarkGzip 650795967 650152275 -0.10%
BenchmarkGunzip 140962363 141041670 +0.06%
BenchmarkHTTPClientServer 71581 73081 +2.10%
BenchmarkJSONEncode 31928079 31913356 -0.05%
BenchmarkJSONDecode 117470065 113689916 -3.22%
BenchmarkMandelbrot200 6008923 5998712 -0.17%
BenchmarkGoParse 6310917 6327487 +0.26%
BenchmarkRegexpMatchMedium_1K 114568 114763 +0.17%
BenchmarkRegexpMatchHard_1K 168977 169244 +0.16%
BenchmarkRevcomp 935294971 914060918 -2.27%
BenchmarkTemplate 145917123 148186096 +1.55%
Minux previous reported larger variations, but these were caused by
run-to-run noise, not repeatable slowdowns.
Actual code changes by Minux.
I only did the docs and the benchmarking.
LGTM=dvyukov, iant, minux
R=minux, josharian, iant, dave, bradfitz, dvyukov
CC=golang-codereviews
https://golang.org/cl/109050043
Also implement go:nosplit annotation. Not really needed
for now, but we'll definitely need it for other conversions.
benchmark old ns/op new ns/op delta
BenchmarkRuneIterate 534 474 -11.24%
BenchmarkRuneIterate2 535 470 -12.15%
LGTM=bradfitz
R=golang-codereviews, dave, bradfitz, minux
CC=golang-codereviews
https://golang.org/cl/93380044
The monotonic clock patch changed all runtime times
to abstract monotonic time. As the result user-visible
MemStats.LastGC become monotonic time as well.
Restore Unix time for LastGC.
This is the simplest way to expose time.now to runtime that I found.
Another option would be to change time.now to C called
int64 runtime.unixnanotime() and then express time.now in terms of it.
But this would require to introduce 2 64-bit divisions into time.now.
Another option would be to change time.now to C called
void runtime.unixnanotime1(struct {int64 sec, int32 nsec} *now)
and then express both time.now and runtime.unixnanotime in terms of it.
Fixes#7852.
LGTM=minux.ma, iant
R=minux.ma, rsc, iant
CC=golang-codereviews
https://golang.org/cl/93720045
Given
type Outer struct {
*Inner
...
}
the compiler generates the implementation of (*Outer).M dispatching to
the embedded Inner. The implementation is logically:
func (p *Outer) M() {
(p.Inner).M()
}
but since the only change here is the replacement of one pointer
receiver with another, the actual generated code overwrites the
original receiver with the p.Inner pointer and then jumps to the M
method expecting the *Inner receiver.
During reflect.Value.Call, we create an argument frame and the
associated data structures to describe it to the garbage collector,
populate the frame, call reflect.call to run a function call using
that frame, and then copy the results back out of the frame. The
reflect.call function does a memmove of the frame structure onto the
stack (to set up the inputs), runs the call, and the memmoves the
stack back to the frame structure (to preserve the outputs).
Originally reflect.call did not distinguish inputs from outputs: both
memmoves were for the full stack frame. However, in the case where the
called function was one of these wrappers, the rewritten receiver is
almost certainly a different type than the original receiver. This is
not a problem on the stack, where we use the program counter to
determine the type information and understand that during (*Outer).M
the receiver is an *Outer while during (*Inner).M the receiver in the
same memory word is now an *Inner. But in the statically typed
argument frame created by reflect, the receiver is always an *Outer.
Copying the modified receiver pointer off the stack into the frame
will store an *Inner there, and then if a garbage collection happens
to scan that argument frame before it is discarded, it will scan the
*Inner memory as if it were an *Outer. If the two have different
memory layouts, the collection will intepret the memory incorrectly.
Fix by only copying back the results.
Fixes#7725.
LGTM=khr
R=khr
CC=dave, golang-codereviews
https://golang.org/cl/85180043
REP MOVSQ and REP STOSQ have a really high startup overhead.
Use a Duff's device to do the repetition instead.
benchmark old ns/op new ns/op delta
BenchmarkClearFat32 7.20 1.60 -77.78%
BenchmarkCopyFat32 6.88 2.38 -65.41%
BenchmarkClearFat64 7.15 3.20 -55.24%
BenchmarkCopyFat64 6.88 3.44 -50.00%
BenchmarkClearFat128 9.53 5.34 -43.97%
BenchmarkCopyFat128 9.27 5.56 -40.02%
BenchmarkClearFat256 13.8 9.53 -30.94%
BenchmarkCopyFat256 13.5 10.3 -23.70%
BenchmarkClearFat512 22.3 18.0 -19.28%
BenchmarkCopyFat512 22.0 19.7 -10.45%
BenchmarkCopyFat1024 36.5 38.4 +5.21%
BenchmarkClearFat1024 35.1 35.0 -0.28%
TODO: use for stack frame zeroing
TODO: REP prefixes are still used for "reverse" copying when src/dst
regions overlap. Might be worth fixing.
LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, r
https://golang.org/cl/81370046
Structured Exception Handling (SEH) was the first way to handle
exceptions (memory faults, divides by zero) on Windows.
The S might as well stand for "stack-based": the implementation
interprets stack addresses in a few different ways, and it gets
subtly confused by Go's management of stacks. It's also something
that requires active maintenance during cgo switches, and we've
had bugs in that maintenance in the past.
We have recently come to believe that SEH cannot work with
Go's stack usage. See http://golang.org/issue/7325 for details.
Vectored Exception Handling (VEH) is more like a Unix signal
handler: you set it once for the whole process and forget about it.
This CL drops all the SEH code and replaces it with VEH code.
Many special cases and 7 #ifdefs disappear.
VEH was introduced in Windows XP, so Go on windows/386 will
now require Windows XP or later. The previous requirement was
Windows 2000 or later. Windows 2000 immediately preceded
Windows XP, so Windows 2000 is the only affected version.
Microsoft stopped supporting Windows 2000 in 2010.
See http://golang.org/s/win2000-golang-nuts for details.
Fixes#7325.
LGTM=alex.brainman, r
R=golang-codereviews, alex.brainman, stephen.gutekanst, dave
CC=golang-codereviews, iant, r
https://golang.org/cl/74790043
For non-closure functions, the context register is uninitialized
on entry and will not be used, but morestack saves it and then the
garbage collector treats it as live. This can be a source of memory
leaks if the context register points at otherwise dead memory.
Avoid this by introducing a parallel set of morestack functions
that clear the context register, and use those for the non-closure functions.
I hope this will help with some of the finalizer flakiness, but it probably won't.
Fixes#7244.
LGTM=dvyukov
R=khr, dvyukov
CC=golang-codereviews
https://golang.org/cl/71030044
These previously reviewed CLs are present in this CL.
---
changeset: 18445:436bb084caed
user: Russ Cox <rsc@golang.org>
date: Mon Nov 11 09:50:34 2013 -0500
description:
runtime: assembly and system calls for Native Client x86-64
See golang.org/s/go13nacl for design overview.
This CL is publicly visible but not CC'ed to golang-dev,
to avoid distracting from the preparation of the Go 1.2
release.
This CL and the others will be checked into my rsc-go13nacl
clone repo for now, and I will send CLs against the main
repo early in the Go 1.3 development.
R≡adg
https://golang.org/cl/15760044
---
changeset: 18448:90bd871b5994
user: Russ Cox <rsc@golang.org>
date: Mon Nov 11 09:51:36 2013 -0500
description:
runtime: amd64p32 and Native Client assembly bootstrap
See golang.org/s/go13nacl for design overview.
This CL is publicly visible but not CC'ed to golang-dev,
to avoid distracting from the preparation of the Go 1.2
release.
This CL and the others will be checked into my rsc-go13nacl
clone repo for now, and I will send CLs against the main
repo early in the Go 1.3 development.
R≡khr
https://golang.org/cl/15820043
---
changeset: 18449:b011c3dc687e
user: Russ Cox <rsc@golang.org>
date: Mon Nov 11 09:51:58 2013 -0500
description:
math: amd64p32 assembly routines
These routines only manipulate float64 values,
so the amd64 and amd64p32 can share assembly.
The large number of files is symptomatic of a problem
with package path: it is a Go package structured like a C library.
But that will need to wait for another day.
See golang.org/s/go13nacl for design overview.
This CL is publicly visible but not CC'ed to golang-dev,
to avoid distracting from the preparation of the Go 1.2
release.
This CL and the others will be checked into my rsc-go13nacl
clone repo for now, and I will send CLs against the main
repo early in the Go 1.3 development.
R≡bradfitz
https://golang.org/cl/15870043
---
changeset: 18450:43234f082eec
user: Russ Cox <rsc@golang.org>
date: Mon Nov 11 10:03:19 2013 -0500
description:
syscall: networking for Native Client
See golang.org/s/go13nacl for design overview.
This CL is publicly visible but not CC'ed to golang-dev,
to avoid distracting from the preparation of the Go 1.2
release.
This CL and the others will be checked into my rsc-go13nacl
clone repo for now, and I will send CLs against the main
repo early in the Go 1.3 development.
R≡rsc
https://golang.org/cl/15780043
---
changeset: 18451:9c8d1d890aaa
user: Russ Cox <rsc@golang.org>
date: Mon Nov 11 10:03:34 2013 -0500
description:
runtime: assembly and system calls for Native Client x86-32
See golang.org/s/go13nacl for design overview.
This CL is publicly visible but not CC'ed to golang-dev,
to avoid distracting from the preparation of the Go 1.2
release.
This CL and the others will be checked into my rsc-go13nacl
clone repo for now, and I will send CLs against the main
repo early in the Go 1.3 development.
R≡rsc
https://golang.org/cl/15800043
---
changeset: 18452:f90b1dd9228f
user: Russ Cox <rsc@golang.org>
date: Mon Nov 11 11:04:09 2013 -0500
description:
runtime: fix frame size for linux/amd64 runtime.raise
R≡rsc
https://golang.org/cl/24480043
---
changeset: 18445:436bb084caed
user: Russ Cox <rsc@golang.org>
date: Mon Nov 11 09:50:34 2013 -0500
description:
runtime: assembly and system calls for Native Client x86-64
See golang.org/s/go13nacl for design overview.
This CL is publicly visible but not CC'ed to golang-dev,
to avoid distracting from the preparation of the Go 1.2
release.
This CL and the others will be checked into my rsc-go13nacl
clone repo for now, and I will send CLs against the main
repo early in the Go 1.3 development.
R≡adg
https://golang.org/cl/15760044
---
changeset: 18455:53b06799a938
user: Russ Cox <rsc@golang.org>
date: Mon Nov 11 23:29:52 2013 -0500
description:
cmd/gc: add -nolocalimports flag
R≡dsymonds
https://golang.org/cl/24990043
---
changeset: 18456:24f64e1eaa8a
user: Russ Cox <rsc@golang.org>
date: Tue Nov 12 22:06:29 2013 -0500
description:
runtime: add comments for playback write
R≡adg
https://golang.org/cl/25190043
---
changeset: 18457:d1f615bbb6e4
user: Russ Cox <rsc@golang.org>
date: Wed Nov 13 17:03:52 2013 -0500
description:
runtime: write only to NaCl stdout, never to NaCl stderr
NaCl writes some other messages on standard error
that we would like to be able to squelch.
R≡adg
https://golang.org/cl/26240044
---
changeset: 18458:1f01be1a1dc2
tag: tip
user: Russ Cox <rsc@golang.org>
date: Wed Nov 13 19:45:16 2013 -0500
description:
runtime: remove apparent debugging dreg
Setting timens to 0 turns off fake time.
TBR≡adg
https://golang.org/cl/26400043
LGTM=bradfitz
R=dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/68730043
Introduces two-phase goroutine parking mechanism -- prepare to park, commit park.
This mechanism does not require backing mutex to protect wait predicate.
Use it in netpoll. See comment in netpoll.goc for details.
This slightly reduces contention between reader, writer and read/write io notifications;
and just eliminates a bunch of mutex operations from hotpaths, thus making then faster.
benchmark old ns/op new ns/op delta
BenchmarkTCP4ConcurrentReadWrite 2109 1945 -7.78%
BenchmarkTCP4ConcurrentReadWrite-2 1162 1113 -4.22%
BenchmarkTCP4ConcurrentReadWrite-4 798 755 -5.39%
BenchmarkTCP4ConcurrentReadWrite-8 803 748 -6.85%
BenchmarkTCP4Persistent 9411 9240 -1.82%
BenchmarkTCP4Persistent-2 5888 5813 -1.27%
BenchmarkTCP4Persistent-4 4016 3968 -1.20%
BenchmarkTCP4Persistent-8 3943 3857 -2.18%
R=golang-codereviews, mikioh.mikioh, gobot, iant, rsc
CC=golang-codereviews, khr
https://golang.org/cl/45700043
Bug #1:
Issue 5406 identified an interesting case:
defer iface.M()
may end up calling a wrapper that copies an indirect receiver
from the iface value and then calls the real M method. That's
two calls down, not just one, and so recover() == nil always
in the real M method, even during a panic.
[For the purposes of this entire discussion, a wrapper's
implementation is a function containing an ordinary call, not
the optimized tail call form that is somtimes possible. The
tail call does not create a second frame, so it is already
handled correctly.]
Fix this bug by introducing g->panicwrap, which counts the
number of bytes on current stack segment that are due to
wrapper calls that should not count against the recover
check. All wrapper functions must now adjust g->panicwrap up
on entry and back down on exit. This adds slightly to their
expense; on the x86 it is a single instruction at entry and
exit; on the ARM it is three. However, the alternative is to
make a call to recover depend on being able to walk the stack,
which I very much want to avoid. We have enough problems
walking the stack for garbage collection and profiling.
Also, if performance is critical in a specific case, it is already
faster to use a pointer receiver and avoid this kind of wrapper
entirely.
Bug #2:
The old code, which did not consider the possibility of two
calls, already contained a check to see if the call had split
its stack and so the panic-created segment was one behind the
current segment. In the wrapper case, both of the two calls
might split their stacks, so the panic-created segment can be
two behind the current segment.
Fix this by propagating the Stktop.panic flag forward during
stack splits instead of looking backward during recover.
Fixes#5406.
R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13367052
This replaces the mcall frame with the badmcall frame instead of
leaving the mcall frame on the stack and adding the badmcall frame.
Because mcall is no longer on the stack, traceback will now report what
called mcall, which is what we would like to see in this situation.
R=golang-dev, cshapiro
CC=golang-dev
https://golang.org/cl/13012044
We can then include this file in assembly to replace
cryptic constants like "7" with meaningful constants
like "(NOPROF|DUPOK|NOSPLIT)".
Converting just pkg/runtime/asm*.s for now. Dropping NOPROF
and DUPOK from lots of places where they aren't needed.
More .s files to come in a subsequent changelist.
A nonzero number in the textflag field now means
"has not been converted yet".
R=golang-dev, daniel.morsing, rsc, khr
CC=golang-dev
https://golang.org/cl/12568043
Per suggestion from Russ in February. Then strings.IndexByte
can be implemented in terms of the shared code in pkg runtime.
Update #3751
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12289043
Debugging the Windows breakage I noticed that SEH
only exists on 386, so we can balance the two stacks
a little more on amd64 and reclaim another word.
Now we're down to just one word consumed by
cgocallback_gofunc, having reclaimed 25% of the
overall budget (4 words out of 16).
Separately, fix windows/386 - the SEH must be on the
m0 stack, as must the saved SP, so we are forced to have
a three-word frame for 386. It matters much less for
386, because there 128 bytes gives 32 words to use.
R=dvyukov, alex.brainman
CC=golang-dev
https://golang.org/cl/11551044
