debug/dwarf: add Reader.SeekPC and Data.Ranges

These new methods help find the compilation unit to pass to the
LineReader method in order to find the line information for a PC.
The Ranges method also helps identify the specific function for a PC,
needed to determine the function name.

This uses the .debug.ranges section if necessary, and changes the object
file format packages to pass in the section contents if available.

Change-Id: I5ebc3d27faaf1a126ffb17a1e6027efdf64af836
Reviewed-on: https://go-review.googlesource.com/20769
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 25 insertions, 0 deletions

diff --git a/src/debug/dwarf/testdata/ranges.c b/src/debug/dwarf/testdata/ranges.c
new file mode 100644
index 0000000000..2f208e591c
--- /dev/null
+++ b/src/debug/dwarf/testdata/ranges.c
@@ -0,0 +1,25 @@
+// gcc -g -O2 -freorder-blocks-and-partition
+
+const char *arr[10000];
+const char *hot = "hot";
+const char *cold = "cold";
+
+__attribute__((noinline))
+void fn(int path) {
+	int i;
+
+	if (path) {
+		for (i = 0; i < sizeof arr / sizeof arr[0]; i++) {
+			arr[i] = hot;
+		}
+	} else {
+		for (i = 0; i < sizeof arr / sizeof arr[0]; i++) {
+			arr[i] = cold;
+		}
+	}
+}
+
+int main(int argc, char *argv[]) {
+	fn(argc);
+	return 0;
+}
diff --git a/src/debug/dwarf/testdata/ranges.elf b/src/debug/dwarf/testdata/ranges.elf
new file mode 100755
index 0000000000..7f54138cff
--- /dev/null
+++ b/src/debug/dwarf/testdata/ranges.elf