Re: [PATCH] Fix gdb.dwarf2/dwz.exp for thumb mode

[Yao Qi <yao@codesourcery.com>]

Yao Qi <yao@codesourcery.com> writes:

> +asm (".section	\".text\"");
> +asm (".balign 8");
> +asm ("main_start: .globl main_start");
> +
>  int
>  main()

Doug expressed the concern that this approach is fragile to clang,
because clang will move the label after function main.  After some
experiments, I find a reliable approach: define a label main_label
inside function main, so neither gcc or clang moves it out of
function.  We can get the address of main by main_label minus the offset
of main_label to main.

In this updated patch, we compile main.c to object, and gets
main_label's offset to main.  Then, we can produce the correct address
of main in the debug information.

-- 
Yao (齐尧)

From: Yao Qi <yao@codesourcery.com>
Subject: [PATCH] Fix gdb.dwarf2/dwz.exp for thumb mode

We see the fail in gdb.dwarf2/dwz.exp in thumb mode,

p the_int^M
$2 = 99^M
(gdb) FAIL: gdb.dwarf2/dwz.exp: p the_int

In thumb mode, the lsb of references to 'main' in the assembly
(produced by dwarf assember) is set, so the generated debug
information is incorrect.

in this patch, we add a label main_label inside function main.  Linker
doesn't set its lsb in thumb mode, and we can get function main's
address by 'main_label - $main_label_offset'.  This label isn't moved
out of function main in both gcc and clang.

gdb/testsuite:

2014-09-13  Yao Qi  <yao@codesourcery.com>

	* gdb.dwarf2/main.c (main): New lable main_label.
	* gdb.dwarf2/dwz.exp: Get the offset of main_label to main.
	Return early if $main_label_offset is unknown.  Get address
	of main by "main_label - $main_label_offset".

diff --git a/gdb/testsuite/gdb.dwarf2/dwz.exp b/gdb/testsuite/gdb.dwarf2/dwz.exp
index 9175f9e..229f512 100644
--- a/gdb/testsuite/gdb.dwarf2/dwz.exp
+++ b/gdb/testsuite/gdb.dwarf2/dwz.exp
@@ -27,8 +27,9 @@ if  {[gdb_compile ${srcdir}/${subdir}/${srcfile} ${binfile}1.o \
     return -1
 }
 
-# Start GDB and load object file, compute the function length which is
-# needed in the Dwarf Assembler below.
+# Start GDB and load object file, compute the function length and the
+# offset from main to main_label which are needed in the Dwarf Assembler
+# below.
 clean_restart ${testfile}1.o
 
 set main_length ""
@@ -59,9 +60,17 @@ gdb_test_multiple $test $test {
     }
 }
 
-if { $main_length == "" } {
+set main_label_offset ""
+set test "p main_label"
+gdb_test_multiple $test $test {
+    -re ".* = {.*} $hex <main\\+($decimal)>.*\r\n$gdb_prompt $" {
+	set main_label_offset $expect_out(1,string)
+    }
+}
+
+if { $main_length == "" || $main_label_offset == "" } {
     # Bail out here, because we can't do the following tests if
-    # $main_length is unknown.
+    # $main_length or $main_label_offset is unknown.
     return -1
 }
 
@@ -71,7 +80,7 @@ gdb_exit
 set asm_file [standard_output_file $srcfile2]
 Dwarf::assemble $asm_file {
     declare_labels partial_label int_label int_label2
-    global main_length
+    global main_length main_label_offset
 
     extern main
 
@@ -79,8 +88,8 @@ Dwarf::assemble $asm_file {
 	partial_label: partial_unit {} {
 	    subprogram {
 		{name main}
-		{low_pc main addr}
-		{high_pc "main + $main_length" addr}
+		{low_pc "main_label - $main_label_offset" addr}
+		{high_pc "main_label - $main_label_offset + $main_length" addr}
 	    }
 	}
     }
diff --git a/gdb/testsuite/gdb.dwarf2/main.c b/gdb/testsuite/gdb.dwarf2/main.c
index 3ddd194..e6d4715 100644
--- a/gdb/testsuite/gdb.dwarf2/main.c
+++ b/gdb/testsuite/gdb.dwarf2/main.c
@@ -20,5 +20,6 @@
 int
 main()
 {
+  asm ("main_label: .globl main_label");
   return 0;
 }