mirror of https://github.com/golang/go.git
46587483e3
When an opcode generates a known high bit state (typically, a sub-word
operation that zeros the high bits), we can remove any subsequent
extension operation that would be a no-op.
x = (OP ...)
y = (ZeroExt32to64 x)
If OP zeros the high 32 bits, then we can replace y with x, as the
zero extension doesn't do anything.
However, x in this situation normally has a sub-word-sized type. The
semantics of values in registers is typically that the high bits
beyond the value's type size are junk. So although the opcode
generating x *currently* zeros the high bits, after x is rewritten to
another opcode it may not - rewrites of sub-word-sized values can
trash the high bits.
To fix, move the extension-removing rules to late lower. That ensures
that their arguments won't be rewritten to change their high bits.
I am also worried about spilling and restoring. Spilling and restoring
doesn't preserve the high bits, but instead sets them to a known value
(often 0, but in some cases it could be sign-extended). I am unable
to come up with a case that would cause a problem here, so leaving for
another time.
Update #66076
Change-Id: I3b5c091b3b3278ccbb7f11beda8b56f4b6d3fde7
Reviewed-on: https://go-review.googlesource.com/c/go/+/568616
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit
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README.md
The test directory contains tests of the Go tool chain and runtime. It includes black box tests, regression tests, and error output tests. They are run as part of all.bash.
To run just these tests, execute:
../bin/go test cmd/internal/testdir
To run just tests from specified files in this directory, execute:
../bin/go test cmd/internal/testdir -run='Test/(file1.go|file2.go|...)'
Standard library tests should be written as regular Go tests in the appropriate package.
The tool chain and runtime also have regular Go tests in their packages. The main reasons to add a new test to this directory are:
- it is most naturally expressed using the test runner; or
- it is also applicable to
gccgoand other Go tool chains.