Add defines for assembly build (#3528)

* Add defines for assembly build

Fixes #1894 by adding GOOS_
GOARCH_ and GOOS_GOARCH_ defines for building assembly.

* Return a copy of args

* Use build.Default variables for os and arch

Add a compilation test that uses another architecture with conditional
inclusion of an assembly function. This mimics a real library that fails
to build under rules_go.

---------

Co-authored-by: Patrick Scott <patrick.scott@observeinc.com>

go/tools/builders/asm.go

@@ -15,6 +15,7 @@
 package main
 
 import (
+	"go/build"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -24,6 +25,12 @@ import (
 	"strings"
 )
 
+var ASM_DEFINES = []string{
+	"-D", "GOOS_" + build.Default.GOOS,
+	"-D", "GOARCH_" + build.Default.GOARCH,
+	"-D", "GOOS_GOARCH_" + build.Default.GOOS + "_" + build.Default.GOARCH,
+}
+
 // buildSymabisFile generates a file from assembly files that is consumed
 // by the compiler. This is only needed in go1.12+ when there is at least one
 // .s file. If the symabis file is not needed, no file will be generated,
@@ -87,8 +94,7 @@ func buildSymabisFile(goenv *env, sFiles, hFiles []fileInfo, asmhdr string) (str
 			seenHdrDirs[hdrDir] = true
 		}
 	}
-	// TODO(#1894): define GOOS_goos, GOARCH_goarch, both here and in the
-	// GoAsm action.
+	asmargs = append(asmargs, ASM_DEFINES...)
 	asmargs = append(asmargs, "-gensymabis", "-o", symabisName, "--")
 	for _, sFile := range sFiles {
 		asmargs = append(asmargs, sFile.filename)
@@ -107,6 +113,7 @@ func asmFile(goenv *env, srcPath, packagePath string, asmFlags []string, outPath
 	if packagePath != "" && isGo119OrHigher() {
 		args = append(args, "-p", packagePath)
 	}
+	args = append(args, ASM_DEFINES...)
 	args = append(args, "-trimpath", ".")
 	args = append(args, "-o", outPath)
 	args = append(args, "--", srcPath)

tests/core/cross/BUILD.bazel

@@ -33,6 +33,14 @@ go_binary(
     deps = [":platform_lib"],
 )
 
+go_binary(
+    name = "asm_cross",
+    srcs = ["asm.s", "main.go"],
+    goarch = "386",
+    goos = "linux",
+    deps = [":platform_lib"],
+)
+
 go_binary(
     name = "native_bin",
     srcs = ["main.go"],

tests/core/cross/asm.s

@@ -0,0 +1,15 @@
+// Example assembly copied from https://github.com/rpccloud/goid
+
+#include "go_asm.h"
+#include "textflag.h"
+
+#ifdef GOARCH_386
+#define	get_tls(r)	MOVL TLS, r
+#define	g(r)	0(r)(TLS*1)
+#endif
+
+TEXT ·getg(SB), NOSPLIT, $0-4
+    get_tls(CX)
+    MOVL    g(CX), AX
+    MOVL    AX, ret+0(FP)
+    RET