[PATCH 1/4] Prefer symtab symbol over minsym for function names in non-contiguous blocks

[Kevin Buettner]

The discussion on gdb-patches which led to this patch may be found
here:

https://www.sourceware.org/ml/gdb-patches/2019-05/msg00018.html

Here's a brief synopsis/analysis:

Eli Zaretskii, while debugging a Windows emacs executable, found
that functions comprised of more than one (non-contiguous)
address range were not being displayed correctly in a backtrace.  This
is the example that Eli provided:

  (gdb) bt
  #0  0x76a63227 in KERNELBASE!DebugBreak ()
     from C:\Windows\syswow64\KernelBase.dll
  #1  0x012e7b89 in emacs_abort () at w32fns.c:10768
  #2  0x012e1f3b in print_vectorlike.cold () at print.c:1824
  #3  0x011d2dec in print_object (obj=<optimized out>, printcharfun=XIL(0),
      escapeflag=true) at print.c:2150

The function print_vectorlike consists of two address ranges, one of
which contains "cold" code which is expected to not execute very often.
There is a minimal symbol, print_vectorlike.cold.65, which is the address
of the "cold" range.

GDB is prefering this minsym over the the name provided by the
DWARF info due to some really old code in GDB which handles
"certain pathological cases".  See the first big block comment
in find_frame_funname for more information.

I considered removing the code for this corner case entirely, but it
seems as though it might still be useful, so I left it intact.

That code is already disabled for inline functions.  I added a
condition which disables it for non-contiguous functions as well.

The change to find_frame_funname in stack.c fixes this problem for
stack traces.  A similar change to print_address_symbolic in
printcmd.c fixes this problem for the "x" command and other commands
which use print_address_symbolic().

gdb/ChangeLog:

	* stack.c (find_frame_funname): Disable use of minsym for function
	name in functions comprised of non-contiguous blocks.
	* printcmd.c (print_address_symbolic): Likewise.
---
 gdb/printcmd.c |  4 +++-
 gdb/stack.c    | 15 ++++++++++++---
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/gdb/printcmd.c b/gdb/printcmd.c
index 9e84594fe6..e00a9c671a 100644
--- a/gdb/printcmd.c
+++ b/gdb/printcmd.c
@@ -627,7 +627,9 @@ build_address_symbolic (struct gdbarch *gdbarch,
 
   if (msymbol.minsym != NULL)
     {
-      if (BMSYMBOL_VALUE_ADDRESS (msymbol) > name_location || symbol == NULL)
+      if (symbol == NULL
+          || (BLOCK_CONTIGUOUS_P (SYMBOL_BLOCK_VALUE (symbol))
+	      && BMSYMBOL_VALUE_ADDRESS (msymbol) > name_location))
 	{
 	  /* If this is a function (i.e. a code address), strip out any
 	     non-address bits.  For instance, display a pointer to the
diff --git a/gdb/stack.c b/gdb/stack.c
index 408c795e38..d511690a14 100644
--- a/gdb/stack.c
+++ b/gdb/stack.c
@@ -1067,9 +1067,18 @@ find_frame_funname (struct frame_info *frame, enum language *funlang,
 
       struct bound_minimal_symbol msymbol;
 
-      /* Don't attempt to do this for inlined functions, which do not
-	 have a corresponding minimal symbol.  */
-      if (!block_inlined_p (SYMBOL_BLOCK_VALUE (func)))
+      /* Don't attempt to do this for two cases:
+
+	 1) Inlined functions, which do not have a corresponding minimal
+	    symbol.
+
+	 2) Functions which are comprised of non-contiguous blocks.
+	    Such functions often contain a minimal symbol for a
+	    "cold" range, i.e. code which is not expected to execute
+	    very often.  It is incorrect to use the minimal symbol
+	    associated with this range.  */
+      if (!block_inlined_p (SYMBOL_BLOCK_VALUE (func))
+          && BLOCK_CONTIGUOUS_P (SYMBOL_BLOCK_VALUE (func)))
 	msymbol
 	  = lookup_minimal_symbol_by_pc (get_frame_address_in_block (frame));
       else
-- 
2.21.0