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Re: Sequel to PR40521 -- -g causes GCC to generate .eh_frame


On Thu, Mar 25, 2010 at 02:34:26PM +0100, Thomas Schwinge wrote:
> Hello!
> 
> Here is a sequel to PR40521 -- -g causes GCC to generate .eh_frame.
> 
> On arm-none-linux-gnueabi, if -fexceptions /
> -fasynchronous-unwind-tables / -funwind-tables is in effect, GCC will
> now *always* emit CFI statements -- and .cfi_sections .debug_frame
> additionally *only* if -g is specified.

This is IMO a serious problem; it generates a lot of extra data for
C++ on ARM (a primary platform).

I couldn't figure out the existing logic, so I've simplified it and
attached another possible solution (untested).  It does have some
related changes in behavior, because the existing checks are
inconsistent; some are:

  flag_unwind_tables || (flag_exceptions && ! USING_SJLJ_EXCEPTIONS),

Others are:

  ! USING_SJLJ_EXCEPTIONS && (flag_unwind_tables || flag_exceptions)

> Defining gcc/config/arm/arm.h:DWARF2_UNWIND_INFO to 0 instead of 1 has
> the desired effect, but I'm not really sure that is correct to do?

By comparison with IA64, I think this is (surprisingly, to me) the
right solution.  In fact, delete the definition.  Then non-EABI ARM
configurations will allow DWARF2_UNWIND_INFO via defaults.h.

Thomas, want to give this a spin?

-- 
Daniel Jacobowitz
CodeSourcery

2010-03-30  Daniel Jacobowitz  <dan@codesourcery.com>

	* dwarf2out.c (NEED_UNWIND_TABLES): Define.
	(dwarf2out_do_frame, dwarf2out_do_cfi_asm, dwarf2out_begin_prologue)
	(dwarf2out_frame_finish): Use NEED_UNWIND_TABLES.
	(dwarf2out_assembly_start): Likewise.  Assert if we are generating
	DWARF2 unwind tables when TARGET_UNWIND_INFO is defined.
	* config/arm/arm.h (DWARF2_UNWIND_INFO): Delete definition.

Index: dwarf2out.c
===================================================================
--- dwarf2out.c	(revision 157825)
+++ dwarf2out.c	(working copy)
@@ -124,6 +124,9 @@ int vms_file_stats_name (const char *, l
 # endif
 #endif
 
+#define NEED_UNWIND_TABLES \
+  (flag_unwind_tables || (flag_exceptions && ! USING_SJLJ_EXCEPTIONS))
+
 /* Map register numbers held in the call frame info that gcc has
    collected using DWARF_FRAME_REGNUM to those that should be output in
    .debug_frame and .eh_frame.  */
@@ -147,9 +150,7 @@ dwarf2out_do_frame (void)
 	  || write_symbols == VMS_AND_DWARF2_DEBUG
 	  || DWARF2_FRAME_INFO || saved_do_cfi_asm
 #ifdef DWARF2_UNWIND_INFO
-	  || (DWARF2_UNWIND_INFO
-	      && (flag_unwind_tables
-		  || (flag_exceptions && ! USING_SJLJ_EXCEPTIONS)))
+	  || (DWARF2_UNWIND_INFO && NEED_UNWIND_TABLES)
 #endif
 	  );
 }
@@ -185,7 +186,7 @@ dwarf2out_do_cfi_asm (void)
 #ifdef TARGET_UNWIND_INFO
       return false;
 #else
-      if (USING_SJLJ_EXCEPTIONS || (!flag_unwind_tables && !flag_exceptions))
+      if (!NEED_UNWIND_TABLES)
 	return false;
 #endif
     }
@@ -3905,8 +3906,7 @@ dwarf2out_begin_prologue (unsigned int l
   /* ??? current_function_func_begin_label is also used by except.c
      for call-site information.  We must emit this label if it might
      be used.  */
-  if ((! flag_exceptions || USING_SJLJ_EXCEPTIONS)
-      && ! dwarf2out_do_frame ())
+  if (! NEED_UNWIND_TABLES && ! dwarf2out_do_frame ())
     return;
 #else
   if (! dwarf2out_do_frame ())
@@ -4066,7 +4066,7 @@ dwarf2out_frame_finish (void)
 
 #ifndef TARGET_UNWIND_INFO
   /* Output another copy for the unwinder.  */
-  if (! USING_SJLJ_EXCEPTIONS && (flag_unwind_tables || flag_exceptions))
+  if (NEED_UNWIND_TABLES)
     output_call_frame_info (1);
 #endif
 }
@@ -20644,9 +20644,11 @@ dwarf2out_assembly_start (void)
 {
   if (HAVE_GAS_CFI_SECTIONS_DIRECTIVE && dwarf2out_do_cfi_asm ())
     {
-#ifndef TARGET_UNWIND_INFO
-      if (USING_SJLJ_EXCEPTIONS || (!flag_unwind_tables && !flag_exceptions))
+#ifdef TARGET_UNWIND_INFO
+      /* We should only be generating .debug_frame.  */
+      gcc_assert (!NEED_UNWIND_TABLES);
 #endif
+      if (!NEED_UNWIND_TABLES)
 	fprintf (asm_out_file, "\t.cfi_sections\t.debug_frame\n");
     }
 }
Index: config/arm/arm.h
===================================================================
--- config/arm/arm.h	(revision 157391)
+++ config/arm/arm.h	(working copy)
@@ -918,9 +918,6 @@ extern int arm_structure_size_boundary;
 #define MUST_USE_SJLJ_EXCEPTIONS 1
 #endif
 
-/* We can generate DWARF2 Unwind info, even though we don't use it.  */
-#define DWARF2_UNWIND_INFO 1
-
 /* Use r0 and r1 to pass exception handling information.  */
 #define EH_RETURN_DATA_REGNO(N) (((N) < 2) ? N : INVALID_REGNUM)
 


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