mirror of
https://github.com/golang/go
synced 2024-09-25 13:20:13 -06:00
The Go programming language
4a8cb4a49c
The extra-clever code in Sincos is trying to do if v&2 == 0 { mask = 0xffffffffffffffff } else { mask = 0 } It does this by turning v&2 into a float64 X0 and then using MOVSD $0.0, X3 CMPSD X0, X3, 0 That CMPSD is defined to behave like: if X0 == X3 { X3 = 0xffffffffffffffff } else { X3 = 0 } which gives the desired mask in X3. The goal in using the CMPSD was to avoid a conditional branch. This code fails when called from a PortAudio callback. In particular, the failure behavior is exactly as if the CMPSD always chose the 'true' execution. Notice that the comparison X0 == X3 is comparing as floating point values the 64-bit pattern v&2 and the actual floating point value zero. The only possible values for v&2 are 0x0000000000000000 (floating point zero) and 0x0000000000000002 (floating point 1e-323, a denormal). If they are both comparing equal to zero, I conclude that in a PortAudio callback (whatever that means), the processor is running in "denormals are zero" mode. I confirmed this by placing the processor into that mode and running the test case in the bug; it produces the incorrect output reported in the bug. In general, if a Go program changes the floating point math modes to something other than what Go expects, the math library is not going to work exactly as intended, so we might be justified in not fixing this at all. However, it seems reasonable that the client code might have expected "denormals are zero" mode to only affect actual processing of denormals. This code has produced what is in effect a gratuitous denormal by being extra clever. There is nothing about the computation being requested that fundamentally requires a denormal. It is also easy to do this computation in integer math instead: mask = ((v&2)>>1)-1 Do that. For the record, the other math tests that fail if you put the processor in "denormals are zero" mode are the tests for Frexp, Ilogb, Ldexp, Logb, Log2, and FloatMinMax, but all fail processing denormal inputs. Sincos was the only function for which that mode causes incorrect behavior on non-denormal inputs. The existing tests check that the new assembly is correct. There is no test for behavior in "denormals are zero" mode, because I don't want to add assembly to change that. Fixes #8623. LGTM=josharian R=golang-codereviews, josharian CC=golang-codereviews, iant, r https://golang.org/cl/151750043 |
||
---|---|---|
api | ||
doc | ||
include | ||
lib | ||
misc | ||
src | ||
test | ||
.hgignore | ||
.hgtags | ||
AUTHORS | ||
CONTRIBUTORS | ||
favicon.ico | ||
LICENSE | ||
PATENTS | ||
README | ||
robots.txt |
This is the source code repository for the Go programming language. For documentation about how to install and use Go, visit http://golang.org/ or load doc/install-source.html in your web browser. After installing Go, you can view a nicely formatted doc/install-source.html by running godoc --http=:6060 and then visiting http://localhost:6060/doc/install/source. Unless otherwise noted, the Go source files are distributed under the BSD-style license found in the LICENSE file. -- Binary Distribution Notes If you have just untarred a binary Go distribution, you need to set the environment variable $GOROOT to the full path of the go directory (the one containing this README). You can omit the variable if you unpack it into /usr/local/go, or if you rebuild from sources by running all.bash (see doc/install.html). You should also add the Go binary directory $GOROOT/bin to your shell's path. For example, if you extracted the tar file into $HOME/go, you might put the following in your .profile: export GOROOT=$HOME/go export PATH=$PATH:$GOROOT/bin See doc/install.html for more details.