Let's say I have a golang package, which contains some assembly code:
demopkg/
source1.go
source2.go
asm_amd64.s
If I try to build it using go build, toolchain will use go tool asm to assemble the *.s files.
But if I add Cgo to the mixture, by putting a single import "C" into any of the sources, go will switch to gcc assembler.
I can see it by executing go build -n. Calls to the /usr/local/go/pkg/tool/linux_amd64/asm from the first case get replaced by calls to gcc. Besides that, it starts complaining about broken syntax.
Is this behaviour documented, so I can rely on it for the maintaining of my package? Can I force go build to use one exact assembler?
Yes, it's in the cgo documentation
When the Go tool sees that one or more Go files use the special import "C", it will look for other non-Go files in the directory and compile them as part of the Go package. Any .c, .s, or .S files will be compiled with the C compiler. Any .cc, .cpp, or .cxx files will be compiled with the C++ compiler. Any .h, .hh, .hpp, or .hxx files will not be compiled separately, but, if these header files are changed, the C and C++ files will be recompiled. The default C and C++ compilers may be changed by the CC and CXX environment variables, respectively; those environment variables may include command line options.