[RFA] xcoffread.c: handle -bbigtoc binaries

[RFA] xcoffread.c: handle -bbigtoc binaries

[Nicholas Duffek]

This patch prevents "pc 0x... in read in psymtab, but not in symtab"
warnings when debugging binaries linked with -bbigtoc.

Explanation:

-bbigtoc binaries have extra sections named @FIX1 containing
linker-generated code to perform data loads from distant locations.  E.g.,
if 'foo' in the following instruction:

  lwz r9,foo(r2)

is greater than 32767, the linker might replace the lwz with a branch to
linker-generated code that does the load in 2 instructions and then
branches back to where execution should continue.

When xcoffread.c encounters such a section, it allocates a new psymtab for
the range of linker-generated instructions contained in the section.
However, the section doesn't associate those instructions with any
functions.  Consequently, when GDB tries to resolve one of those
instructions to a symbolic name, it fails, resulting in the "pc 0x... in
read in psymtab, but not in symtab" warning.

The appended patch avoids that warning by ignoring section names beginning
with '@'.

ChangeLog:

	* xcoffread.c (scan_xcoff_symtab): Ignore symbols beginning with
	"@".

Tested on powerpc-ibm-aix4.3.3.0.  Okay to apply?

Nicholas Duffek
<nsd@redhat.com>

[patch follows]

Index: gdb/xcoffread.c
===================================================================
diff -up gdb/xcoffread.c gdb/xcoffread.c
--- gdb/xcoffread.c	Thu Apr 12 14:39:21 2001
+++ gdb/xcoffread.c	Thu Apr 12 14:39:09 2001
@@ -2243,8 +2243,14 @@ scan_xcoff_symtab (struct objfile *objfi
 	    else
 	      csect_aux = main_aux[0];
 
-	    /* If symbol name starts with ".$" or "$", ignore it.  */
-	    if (namestring[0] == '$'
+	    /* If symbol name starts with ".$" or "$", ignore it. 
+
+	       A symbol like "@FIX1" introduces a section for -bbigtoc jump
+	       tables, which contain anonymous linker-generated code. 
+	       Ignore those sections to avoid "pc 0x... in read in psymtab,
+	       but not in symtab" warnings from find_pc_sect_symtab.  */
+
+	    if (namestring[0] == '$' || namestring[0] == '@'
 		|| (namestring[0] == '.' && namestring[1] == '$'))
 	      break;
 

Re: [RFA] xcoffread.c: handle -bbigtoc binaries

[Kevin Buettner]

On Apr 12,  2:57pm, Nicholas Duffek wrote:

> 	* xcoffread.c (scan_xcoff_symtab): Ignore symbols beginning with
> 	"@".
> 
> Tested on powerpc-ibm-aix4.3.3.0.  Okay to apply?

Yes.  Please check it in.

Kevin

Re: [RFA] xcoffread.c: handle -bbigtoc binaries

[Nick Duffek]

On 12-Apr-2001, Kevin Buettner wrote:

>Yes.  Please check it in.

Thanks, I've committed it.

Nick

Re: [RFA] xcoffread.c: handle -bbigtoc binaries

[Andrew Cagney]

Nick Duffek wrote:
> 
> On 12-Apr-2001, Kevin Buettner wrote:
> 
> >Yes.  Please check it in.
> 
> Thanks, I've committed it.

Kevin, I guess you're calling this an obvious fix?  Anyway, the comment:

+           /* If symbol name starts with ".$" or "$", ignore it. 

doesn't match the code.

+           if (namestring[0] == '$' || namestring[0] == '@'

	Andrew

Re: [RFA] xcoffread.c: handle -bbigtoc binaries

[Kevin Buettner]

On Apr 12,  5:13pm, Andrew Cagney wrote:

> Kevin, I guess you're calling this an obvious fix?

No.  I don't regard this as obvious.  In fact, when I first heard of
this change (a couple of days ago), I had some concerns.  But I've
been following some internal (Red Hat) discussion regarding this
change and it seems okay to me now.

I was approving Nick's patch as one of the AIX maintainers.  If I've
stepped on someone's toes, I apologize.  If someone feels that this
change demands more discussion, I'll ask Nick to revert it until a
consensus is reached.

> Anyway, the comment:
> 
> +           /* If symbol name starts with ".$" or "$", ignore it. 
> 
> doesn't match the code.
> 
> +           if (namestring[0] == '$' || namestring[0] == '@'

If you read the entire comment and the entire if-condition, I believe
it's covered...

	    /* If symbol name starts with ".$" or "$", ignore it. 

	       A symbol like "@FIX1" introduces a section for -bbigtoc jump
	       tables, which contain anonymous linker-generated code. 
	       Ignore those sections to avoid "pc 0x... in read in psymtab,
	       but not in symtab" warnings from find_pc_sect_symtab.  */

	    if (namestring[0] == '$' || namestring[0] == '@'
		|| (namestring[0] == '.' && namestring[1] == '$'))
	      break;


Though I admit that when I first read the patch, I was puzzled too. 
Perhaps it would be clearer if the first line of the comment were
revised to read as follows...

	    /* If symbol name starts with ".$", "$", or "@", ignore it. 

?

BTW, I should add that I appreciate Nick's additional commentary
regarding the reason for ignoring the "@" symbols.  I only wish the
author of the original comment would've done the same for the ".$" and
"$" prefixes.

Kevin