cmd/ld: support for linking with host linker

A step toward a fix for issue 4069.

To allow linking with arbitrary host object files, add a linker mode
that can generate a host object file instead of an executable.
Then the host linker can be invoked to generate the final executable.

This CL adds a new -hostobj flag that instructs the linker to write
a host object file instead of an executable.

That is, this works:

        go tool 6g x.go
        go tool 6l -hostobj -o x.o x.6
        ld -e _rt0_amd64_linux x.o
        ./a.out

as does:

        go tool 8g x.go
        go tool 8l -hostld ignored -o x.o x.8
        ld -m elf_i386 -e _rt0_386_linux x.o
        ./a.out

Because 5l was never updated to use the standard relocation scheme,
it will take more work to get this working on ARM.

This is a checkpoint of the basic functionality. It does not work
with cgo yet, and cgo is the main reason for the change.
The command-line interface will likely change too.
The gc linker has other information that needs to be returned to
the caller for use when invoking the host linker besides the single
object file.

R=iant, iant
CC=golang-dev
https://golang.org/cl/7060044

1 files changed, 20 insertions, 2 deletions

diff --git a/src/cmd/ld/data.c b/src/cmd/ld/data.c
index 30e1309d24..e557881e94 100644
--- a/src/cmd/ld/data.c
+++ b/src/cmd/ld/data.c
@@ -146,6 +146,7 @@ void
 relocsym(Sym *s)
 {
 	Reloc *r;
+	Sym *rs;
 	Prog p;
 	int32 i, off, siz, fl;
 	vlong o;
@@ -176,18 +177,35 @@ relocsym(Sym *s)
 		switch(r->type) {
 		default:
 			o = 0;
-			if(archreloc(r, s, &o) < 0)
+			if(isobj || archreloc(r, s, &o) < 0)
 				diag("unknown reloc %d", r->type);
 			break;
 		case D_ADDR:
 			o = symaddr(r->sym) + r->add;
+			if(isobj && r->sym->type != SCONST) {
+				if(thechar == '6')
+					o = 0;
+				else {
+					// set up addend for eventual relocation via outer symbol
+					rs = r->sym;
+					while(rs->outer != nil)
+						rs = rs->outer;
+					o -= symaddr(rs);
+				}
+			}
 			break;
 		case D_PCREL:
-			// r->sym can be null when CALL $(constant) is transformed from absoulte PC to relative PC call.
+			// r->sym can be null when CALL $(constant) is transformed from absolute PC to relative PC call.
 			o = 0;
 			if(r->sym)
 				o += symaddr(r->sym);
 			o += r->add - (s->value + r->off + r->siz);
+			if(isobj && r->sym->type != SCONST) {
+				if(thechar == '6')
+					o = 0;
+				else
+					o = r->add - r->siz;
+			}
 			break;
 		case D_SIZE:
 			o = r->sym->size + r->add;