Go patch committed: Look for tool build ID before hashing entire file

[Ian Lance Taylor]

This libgo patch changes the go tool to look for a tool build ID
before hashing the entire file  It also fixes the key used to store
the ID.  This is a significant speedup in go tool run time.
Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 275558)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-5c3f52ffbae7a9bb59bce63cd2cffdd8af8f4a92
+68038b4fdf1456482af986cb05dcf3121bd43ffc
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: libgo/go/cmd/go/internal/work/buildid.go
===================================================================
--- libgo/go/cmd/go/internal/work/buildid.go    (revision 275473)
+++ libgo/go/cmd/go/internal/work/buildid.go    (working copy)
@@ -291,14 +291,19 @@ func (b *Builder) gccgoToolID(name, lang
                                exe = lp
                        }
                }
-               if _, err := os.Stat(exe); err != nil {
-                       return "", fmt.Errorf("%s: can not find compiler %q: 
%v; output %q", name, exe, err, out)
+               id, err = buildid.ReadFile(exe)
+               if err != nil {
+                       return "", err
+               }
+
+               // If we can't find a build ID, use a hash.
+               if id == "" {
+                       id = b.fileHash(exe)
                }
-               id = b.fileHash(exe)
        }
 
        b.id.Lock()
-       b.toolIDCache[name] = id
+       b.toolIDCache[key] = id
        b.id.Unlock()
 
        return id, nil