[release-branch.go1.2] debug/dwarf: add DWARF 4 form constants

««« CL 18460043 / 08e6655618f5
debug/dwarf: add DWARF 4 form constants

Some versions of clang generate DWARF 4-format attributes
even when using -gdwarf-2. We don't care much about the
values, but we do need to be able to parse past them.

This fixes a bug in Go 1.2 rc2 reported via private mail using
a near-tip version of clang.

R=golang-dev, iant, dvyukov
CC=golang-dev
https://golang.org/cl/18460043
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/20470045

src/pkg/debug/dwarf/const.go

@@ -207,7 +207,10 @@ const (
 	formRef8        format = 0x14
 	formRefUdata    format = 0x15
 	formIndirect    format = 0x16
+	formSecOffset   format = 0x17
+	formExprloc     format = 0x18
 	formFlagPresent format = 0x19
+	formRefSig8     format = 0x20
 )
 
 // A Tag is the classification (the type) of an Entry.

src/pkg/debug/dwarf/entry.go

@@ -188,6 +188,7 @@ func (b *buf) entry(atab abbrevTable, ubase Offset) *Entry {
 		// flag
 		case formFlag:
 			val = b.uint8() == 1
+		// New in DWARF 4.
 		case formFlagPresent:
 			// The attribute is implicitly indicated as present, and no value is
 			// encoded in the debugging information entry itself.
@@ -236,6 +237,30 @@ func (b *buf) entry(atab abbrevTable, ubase Offset) *Entry {
 				b.err = b1.err
 				return nil
 			}
+
+		// lineptr, loclistptr, macptr, rangelistptr
+		// New in DWARF 4, but clang can generate them with -gdwarf-2.
+		// Section reference, replacing use of formData4 and formData8.
+		case formSecOffset:
+			is64, known := b.format.dwarf64()
+			if !known {
+				b.error("unknown size for DW_FORM_sec_offset")
+			} else if is64 {
+				val = int64(b.uint64())
+			} else {
+				val = int64(b.uint32())
+			}
+
+		// exprloc
+		// New in DWARF 4.
+		case formExprloc:
+			val = b.bytes(int(b.uint()))
+
+		// reference
+		// New in DWARF 4.
+		case formRefSig8:
+			// 64-bit type signature.
+			val = b.uint64()
 		}
 		e.Field[i].Val = val
 	}