When functions are inlined, for instructions in the inlined body, does
-S print the location of the call, or the location of the body? Right
now, we do the former. I'd like to do the latter by default, it makes
much more sense when reading disassembly. With mid-stack inlining
enabled in more cases, this quandry will come up more often.
The original behavior is still available with -S=2. Some tests
use this mode (so they can find assembly generated by a particular
source line).
This helped me with understanding what the compiler was doing
while fixing #29007.
Change-Id: Id14a3a41e1b18901e7c5e460aa4caf6d940ed064
Reviewed-on: https://go-review.googlesource.com/c/153241
Reviewed-by: David Chase <drchase@google.com>
IsSliceInBounds(x, y) asserts that y is not negative, but
there were cases where this is not true. Change code
generation to ensure that this is true when it's not obviously
true. Prove phase cleans a few of these out.
With this change the compiler text section is 0.06% larger,
that is, not very much. Benchmarking still TBD, may need
to wait for access to a benchmarking box (next week).
Also corrected run.go to handle '?' in -update_errors output.
Fixes#28797.
Change-Id: Ia8af90bc50a91ae6e934ef973def8d3f398fac7b
Reviewed-on: https://go-review.googlesource.com/c/152477
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
A little bit of compiler stress testing. Randomize the order
of the values in a block before every phase. This randomization
makes sure that we're not implicitly depending on that order.
Currently the random seed is a hash of the function name.
It provides determinism, but sacrifices some coverage.
Other arrangements are possible (env var, ...) but require
more setup.
Fixes#20178
Change-Id: Idae792a23264bd9a3507db6ba49b6d591a608e83
Reviewed-on: https://go-review.googlesource.com/c/33909
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Currently, both asm and compile have a -symabis flag, but in asm it's
a boolean flag that means to generate a symbol ABIs file and in the
compiler its a string flag giving the path of the symbol ABIs file to
consume. I'm worried about this false symmetry biting us in the
future, so rename asm's flag to -gensymabis.
Updates #27539.
Change-Id: I8b9c18a852d2838099718f8989813f19d82e7434
Reviewed-on: https://go-review.googlesource.com/c/149818
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Move the empty header file created by "builddir", "buildrundir"
directives to t.tempDir. The file was accidentally placed in the
same directory as the source code and this was a vestige of CL 146999.
Fixes#28781
Change-Id: I3d2ada5f9e8bf4ce4f015b9bd379b311592fe3ce
Reviewed-on: https://go-review.googlesource.com/c/149458
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This is a little clearer, and we're about to need the .s file list in
one more place, so this will cut down on duplication.
Change-Id: I4da8bf03a0469fb97565b0841c40d505657b574e
Reviewed-on: https://go-review.googlesource.com/c/146998
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This appears to have simply been an oversight.
Change-Id: Ia5d1309b3ebc99c9abbf0282397693272d8178aa
Reviewed-on: https://go-review.googlesource.com/c/142885
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Also includes a small tweak to test/run.go to allow package names
with Unicode letters (as opposed to just ASCII chars).
Updates #27836
Change-Id: Idbf0bdea24174808cddcb69974dab820eb13e521
Reviewed-on: https://go-review.googlesource.com/138075
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Adds a new build tag "gcflags_noopt" that can be used in test/*.go
tests.
Fixes#27833
Change-Id: I4ea0ccd9e9e58c4639de18645fec81eb24a3a929
Reviewed-on: https://go-review.googlesource.com/136898
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
The codegen testsuite uses regexp to parse the syntax, but it doesn't
have a way to tell line comments containing checks from line comments
containing English sentences. This means that any syntax error (that
is, non-matching regexp) is currently ignored and not reported.
There were some tests in memcombine.go that had an extraneous space
and were thus effectively disabled. It would be great if we could
report it as a syntax error, but for now we just punt and swallow the
spaces as a workaround, to avoid the same mistake again.
Fixes#25452
Change-Id: Ic7747a2278bc00adffd0c199ce40937acbbc9cf0
Reviewed-on: https://go-review.googlesource.com/113835
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Every action has a short annotation.
The errorCheck function has a comment adapted from errchk script.
Removed redundant assigments to tmpDir.
Change-Id: Ifdd1284de046a0ce2aad26bd8da8a8e6a7707a8e
Reviewed-on: https://go-review.googlesource.com/115856
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This CL removes the rundircmpout action completely
because it is not used anywhere.
The run case already looks for output files. Rename the cmpout action
mentioned in tests to the run action and remove "cmpout" from run.go.
Change-Id: I835ceb70082927f8e9360e0ea0ba74f296363ab3
Reviewed-on: https://go-review.googlesource.com/115575
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
To allow testing of fixedbugs/bug345.go in Go,
a new flag -n is introduced. This flag disables setting
of relative path for local imports and imports search path
to current dir, namely -D . -I . are not passed to the compiler.
Error regexps are fixed to allow running the test in temp directory.
This change eliminates the last place where Perl
script "errchk" was used.
Fixes#25586.
Change-Id: If085f466e6955312d77315f96d3ef1cb68495aef
Reviewed-on: https://go-review.googlesource.com/115277
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This change enables bug248 to be tested with Go code.
For that, it adds a flag -1 to error check and run directory
with one package failing compilation prior the last package
which should be run.
Specifically, the "p" package in bug1.go file was renamed into "q"
to compile them in separate steps,
bug2.go and bug3.go files were reordered,
bug2.go was changed into non-main package.
Updates #25586.
Change-Id: Ie47aacd56ebb2ce4eac66c792d1a53e1e30e637c
Reviewed-on: https://go-review.googlesource.com/114818
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
mips64 softfloat support is based on mips implementation and introduces
new enviroment variable GOMIPS64.
GOMIPS64 is a GOARCH=mips64{,le} specific option, for a choice between
hard-float and soft-float. Valid values are 'hardfloat' (default) and
'softfloat'. It is passed to the assembler as
'GOMIPS64_{hardfloat,softfloat}'.
Change-Id: I7f73078627f7cb37c588a38fb5c997fe09c56134
Reviewed-on: https://go-review.googlesource.com/108475
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
On amd64, Ctz must include special handling of zeros.
But the prove pass has enough information to detect whether the input
is non-zero, allowing a more efficient lowering.
Introduce new CtzNonZero ops to capture and use this information.
Benchmark code:
func BenchmarkVisitBits(b *testing.B) {
b.Run("8", func(b *testing.B) {
for i := 0; i < b.N; i++ {
x := uint8(0xff)
for x != 0 {
sink = bits.TrailingZeros8(x)
x &= x - 1
}
}
})
// and similarly so for 16, 32, 64
}
name old time/op new time/op delta
VisitBits/8-8 7.27ns ± 4% 5.58ns ± 4% -23.35% (p=0.000 n=28+26)
VisitBits/16-8 14.7ns ± 7% 10.5ns ± 4% -28.43% (p=0.000 n=30+28)
VisitBits/32-8 27.6ns ± 8% 19.3ns ± 3% -30.14% (p=0.000 n=30+26)
VisitBits/64-8 44.0ns ±11% 38.0ns ± 5% -13.48% (p=0.000 n=30+30)
Fixes#25077
Change-Id: Ie6e5bd86baf39ee8a4ca7cadcf56d934e047f957
Reviewed-on: https://go-review.googlesource.com/109358
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
While writing CL 107315, I went back and forth for the syntax used for
constraints of build environments in which the architecture did not
support varitants ("plan9/amd64" vs "plan9/amd64/"). I eventually
settled for the latter because the code required less heuristics
(think parsing "plan9/386" vs "386/sse2") but there were a few
leftovers in code and comments.
Change-Id: I9d9a008f3814f9a1642609650eb571e7f1a675cf
Reviewed-on: https://go-review.googlesource.com/107338
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This CL makes the codegen testsuite automatically test all
architecture variants for architecture specified in tests. For
instance, if a test file specifies a "arm" test, it will be
automatically run on all GOARM variants (5,6,7), to increase
the coverage.
The CL also introduces a syntax to specify only a specific
variant (eg: "arm/7") in case the test makes sense only there.
The same syntax also allows to specify the operating system
in case it matters (eg: "plan9/386/sse2").
Fixes#24658
Change-Id: I2eba8b918f51bb6a77a8431a309f8b71af07ea22
Reviewed-on: https://go-review.googlesource.com/107315
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This change modifies the codegen test harness driver so that it no
longer modifies the environment GOOS/GOARCH, since that seems to cause
flakiness in other concurrently-running tests.
The change also enables the codegen tests in run.go.
Fixes#24538
Change-Id: I997ac1eb38eb92054efff67fe5c4d3cccc86412b
Reviewed-on: https://go-review.googlesource.com/103455
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Before this change, in case of any failure, asmcheck was
dumping to stderr the whole output of compile -S, which
can be very long if it contains multiple functions.
Make it so it filters the output to only display the
assembly output of functions for which at least one opcode
check failed. This greatly simplifies debugging.
Change-Id: I1bbf54473b8252a3384e2c1dade82d926afc119d
Reviewed-on: https://go-review.googlesource.com/98444
Run-TryBot: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Keith Randall <khr@golang.org>
Currently, the top-level testsuite always uses whatever version
of Go is found in the PATH to execute all the tests. This
forces the developers to tweak the PATH to run the testsuite.
Change it to use the same version of Go used to run run.go.
This allows developers to run the testsuite using the tip
compiler by simply saying "../bin/go run run.go".
I think this is a better solution compared to always forcing
"../bin/go", because it allows developers to run the testsuite
using different Go versions, for instance to check if a new
test is fixed in tip compared to the installed compiler.
Fixes#24217
Change-Id: I41b299c753b6e77c41e28be9091b2b630efea9d2
Reviewed-on: https://go-review.googlesource.com/98439
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
In RET instruction, the operand is the return jump's target,
which should be put in Prog.To.
Add an action "buildrundir" to the test driver, which builds
(compile+assemble+link) the code in a directory and runs the
resulting binary.
Fixes#23838.
Change-Id: I7ebe7eda49024b40a69a24857322c5ca9c67babb
Reviewed-on: https://go-review.googlesource.com/94175
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
This avoid simple bugs like "ADD" matching "FADD". Obviously
"ADD" will still match "ADDQ" so some care is still required
in this regard, but at least a first class of possible errors
is taken care of.
Change-Id: I7deb04c31de30bedac9c026d9889ace4a1d2adcb
Reviewed-on: https://go-review.googlesource.com/97817
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Keith Randall <khr@golang.org>
asmcheck comments now support a compact form of specifying
multiple checks for each platform, using the following syntax:
amd64:"SHL\t[$]4","SHR\t[$]4"
Negative checks are also parsed using the following syntax:
amd64:-"ROR"
though they are still not working.
Moreover, out-of-line comments have been implemented. This
allows to specify asmchecks on comment-only lines, that will
be matched on the first subsequent non-comment non-empty line.
// amd64:"XOR"
// arm:"EOR"
x ^= 1
Change-Id: I110c7462fc6a5c70fd4af0d42f516016ae7f2760
Reviewed-on: https://go-review.googlesource.com/97816
Reviewed-by: Keith Randall <khr@golang.org>
The top-level test harness is modified to support a new kind
of test: "asmcheck". This is meant to replace asm_test.go
as an easier and more readable way to test code generation.
I've added a couple of codegen tests to get initial feedback
on the syntax. I've created them under a common "codegen"
subdirectory, so that it's easier to run them all with
"go run run.go -v codegen".
The asmcheck syntax allows to insert line comments that
can specify a regular expression to match in the assembly code,
for multiple architectures (the testsuite will automatically
build each testfile multiple times, one per mentioned architecture).
Negative matches are unsupported for now, so this cannot fully
replace asm_test yet.
Change-Id: Ifdbba389f01d55e63e73c99e5f5449e642101d55
Reviewed-on: https://go-review.googlesource.com/97355
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
For "type T = U" we were accidentally emitting a #define for "U__size"
instead of "T__size".
Fixes#22877.
Change-Id: I5ed6757d697753ed6d944077c16150759f6e1285
Reviewed-on: https://go-review.googlesource.com/80759
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This cuts 6 seconds off all.bash with the new go command.
Not a ton, but also an easy 6 seconds to grab.
The -tags=use_go_run in the misc/cgo tests is just some
go command flag that will make run.go use go run,
but without making everything look stale.
(Those tests have relative imports,
so go tool compile+link is not enough.)
Change-Id: I43bf4bb661d3adde2b2d4aad5e8f64b97bc69ba9
Reviewed-on: https://go-review.googlesource.com/73994
Reviewed-by: Ian Lance Taylor <iant@golang.org>
If the go install doesn't use the same flags as the main build
it can overwrite the installed standard library, leading to
flakiness and slow future tests.
Force uses of 'go install' etc to propagate $GO_GCFLAGS
or disable them entirely, to avoid problems.
As I understand it, the main place this happens is the ssacheck builder.
If there are other uses that need to run some of the now-disabled
tests we can reenable fixed tests in followup CLs.
Change-Id: Ib860a253539f402f8a96a3c00ec34f0bbf137c9a
Reviewed-on: https://go-review.googlesource.com/74470
Reviewed-by: David Crawshaw <crawshaw@golang.org>
These are used by DIV[U] and MOD[U] assembly instructions.
Add a test in the stdlib so we actually exercise linking
to these routines.
Update #19507
Change-Id: I0d8e19a53e3744abc0c661ea95486f94ec67585e
Reviewed-on: https://go-review.googlesource.com/45703
Reviewed-by: Cherry Zhang <cherryyz@google.com>
The code in #20162 contains an embedded interface.
It didn't get dowidth'd by the frontend,
and during DWARF generation, ngotype asked
for a string description of it,
which triggered a request for the number of fields
in the interface, which triggered a dowidth,
which is disallowed in the backend.
The other changes in this CL are to support the test.
Fixes#20162
Change-Id: I4d0be5bd949c361d4cdc89a8ed28b10977e40cf9
Reviewed-on: https://go-review.googlesource.com/42131
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Previously, we could not run tests with -l=4 on NaCl since the buildrun
action is not supported on NaCl. This lets us run tests with build flags
on NaCl.
Change-Id: I103370c7b823b4ff46f47df97e802da0dc2bc7c3
Reviewed-on: https://go-review.googlesource.com/38170
Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Compiler errors now show the exact line and line byte offset (sometimes
called "column") of where an error occured. For `go tool compile x.go`:
package p
const c int = false
//line foo.go:123
type t intg
reports
x.go:2:7: cannot convert false to type int
foo.go:123[x.go:4:8]: undefined: intg
(Some errors use the "wrong" position for the error message; arguably
the byte offset for the first error should be 15, the position of 'false',
rathen than 7, the position of 'c'. But that is an indepedent issue.)
The byte offset (column) values are measured in bytes; they start at 1,
matching the convention used by editors and IDEs.
Positions modified by //line directives show the line offset only for the
actual source location (in square brackets), not for the "virtual" file and
line number because that code is likely generated and the //line directive
only provides line information.
Because the new format might break existing tools or scripts, printing
of line offsets can be disabled with the new compiler flag -C. We plan
to remove this flag eventually.
Fixes#10324.
Change-Id: I493f5ee6e78457cf7b00025aba6b6e28e50bb740
Reviewed-on: https://go-review.googlesource.com/37970
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Loop breaking with a counter. Benchmarked (see comments),
eyeball checked for sanity on popular loops. This code
ought to handle loops in general, and properly inserts phi
functions in cases where the earlier version might not have.
Includes test, plus modifications to test/run.go to deal with
timeout and killing looping test. Tests broken by the addition
of extra code (branch frequency and live vars) for added
checks turn the check insertion off.
If GOEXPERIMENT=preemptibleloops, the compiler inserts reschedule
checks on every backedge of every reducible loop. Alternately,
specifying GO_GCFLAGS=-d=ssa/insert_resched_checks/on will
enable it for a single compilation, but because the core Go
libraries contain some loops that may run long, this is less
likely to have the desired effect.
This is intended as a tool to help in the study and diagnosis
of GC and other latency problems, now that goal STW GC latency
is on the order of 100 microseconds or less.
Updates #17831.
Updates #10958.
Change-Id: I6206c163a5b0248e3f21eb4fc65f73a179e1f639
Reviewed-on: https://go-review.googlesource.com/33910
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
It appears to be a vestigial holding ground for bugs.
But we have an issue tracker, and #1909 is there and open.
Change-Id: I912ff222a24c51fab483be0c67dad534f5a84488
Reviewed-on: https://go-review.googlesource.com/31859
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
If -s is specified, each file is considered a separate
package even if multiple files have the same package names.
For instance, the action and flag "errorcheckdir -s"
will compile all files in the respective directory as
individual packages.
Change-Id: Ic5c2f9e915a669433f66c2d3fe0ac068227a502f
Reviewed-on: https://go-review.googlesource.com/24313
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Pushed from an old client by mistake. These are the
missing changes.
Change-Id: Ia8d61c5c0bde907369366ea9ea98711823342803
Reviewed-on: https://go-review.googlesource.com/21349
Reviewed-by: Todd Neal <todd@tneal.org>
One intrinsic was needed to help get the very best
performance out of a future GC; as long as that one was
being added, I also added Bswap since that is sometimes
a handy thing to have. I had intended to fill out the
bit-scan intrinsic family, but the mismatch between the
"scan forward" instruction and "count leading zeroes"
was large enough to cause me to leave it out -- it poses
a dilemma that I'd rather dodge right now.
These intrinsics are not exposed for general use.
That's a separate issue requiring an API proposal change
( https://github.com/golang/proposal )
All intrinsics are tested, both that they are substituted
on the appropriate architecture, and that they produce the
expected result.
Change-Id: I5848037cfd97de4f75bdc33bdd89bba00af4a8ee
Reviewed-on: https://go-review.googlesource.com/20564
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This should probably be considered "experimental" at this stage, but
what it needs is feedback from adventurous adopters. I think the data
structure used for describing escape reasons might be extendable to
allow a cleanup of the underlying algorithms, which suffers from
insufficiently separated concerns (the graph does not deal well with
escape level adjustments, so it is augmented by a second custom-walk
portion of the "flood" phase. It would be better to put it all,
including level adjustments, in a single graph structure, and then
simply flood the graph.
Tweaked to avoid allocations in the no-logging case.
Modified run.go to ignore lines with leading "#" in the output (since
it can never match a line), and in -update_errors to ignore leading
tabs in output lines and to normalize embedded filenames.
Currently requires -m -m because otherwise the noise/update
burden for the other escape tests is considerable.
There is a partial test. Existing escape analysis tests seem to
cover all except the panic case and what looks like it might be
unreachable code in escape analysis.
Fixes#10526.
Change-Id: I2524fdec54facae48b00b2548e25d9e46fcaf832
Reviewed-on: https://go-review.googlesource.com/18041
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
The build tags are necessary to keep "go build" in that directory
building only stdio.go, but we have to arrange for test/run.go to
treat them as satisfied.
Fixes#12625.
Change-Id: Iec0cb2fdc2c9b24a4e0530be25e940aa0cc9552e
Reviewed-on: https://go-review.googlesource.com/17454
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This is a bit ugly but it's a useful test. Run go install -buildmode=shared std
and then go run run.go -linkshared (it passes on linux/amd64).
Change-Id: I5684c79cd03817fa1fc399788b7320f8535c08da
Reviewed-on: https://go-review.googlesource.com/16343
Reviewed-by: Russ Cox <rsc@golang.org>
