cmd/link: nice error message on ABI mismatch

Currently, if a symbol is only defined under one ABI and referenced
under another ABI, you simply get a "relocation target X not defined".
This is confusing because it seems like the symbol is defined.

This CL enhances the error message in this case to be "relocation
target X not defined for <ABI> (but is defined for <ABI>)".

For #27539.

Change-Id: If857a1882c3fe9af5346797d5295ca1fe50ae565
Reviewed-on: https://go-review.googlesource.com/c/147159
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/link/internal/ld/link.go

@@ -32,6 +32,7 @@ package ld
 
 import (
 	"bufio"
+	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
 	"cmd/link/internal/sym"
@@ -108,9 +109,27 @@ func (ctxt *Link) ErrorUnresolved(s *sym.Symbol, r *sym.Reloc) {
 	k := unresolvedSymKey{from: s, to: r.Sym}
 	if !ctxt.unresolvedSymSet[k] {
 		ctxt.unresolvedSymSet[k] = true
+
+		// Try to find symbol under another ABI.
+		var reqABI, haveABI obj.ABI
+		haveABI = ^obj.ABI(0)
+		for abi := obj.ABI(0); abi < obj.ABICount; abi++ {
+			v := sym.ABIToVersion(abi)
+			if v == -1 {
+				continue
+			}
+			if v == int(r.Sym.Version) {
+				reqABI = abi
+			} else if ctxt.Syms.ROLookup(r.Sym.Name, v) != nil {
+				haveABI = abi
+			}
+		}
+
 		// Give a special error message for main symbol (see #24809).
 		if r.Sym.Name == "main.main" {
 			Errorf(s, "function main is undeclared in the main package")
+		} else if haveABI != ^obj.ABI(0) {
+			Errorf(s, "relocation target %s not defined for %s (but is defined for %s)", r.Sym.Name, reqABI, haveABI)
 		} else {
 			Errorf(s, "relocation target %s not defined", r.Sym.Name)
 		}