[PATCH] Use function entry point record only for entry values

[Hannes Domani <ssbssa@yahoo.de>]

PR28987 notes that optimized code sometimes shows the wrong
value of variables at the entry point of a function, if some
code was optimized away and the variable has multiple values
stored in the debug info for this location:
```
(gdb) info address i
Symbol "i" is multi-location:
  Base address 0x140001600  Range 0x13fd41600-0x13fd41600: the constant 0
  Range 0x13fd41600-0x13fd41600: the constant 1
  Range 0x13fd41600-0x13fd41600: the constant 2
  Range 0x13fd41600-0x13fd41600: the constant 3
  Range 0x13fd41600-0x13fd41600: the constant 4
  Range 0x13fd41600-0x13fd41600: the constant 5
  Range 0x13fd41600-0x13fd41600: the constant 6
  Range 0x13fd41600-0x13fd41600: the constant 7
  Range 0x13fd41600-0x13fd4160f: the constant 8
(gdb) p i
$1 = 0
```

Currently, when at the entry point of a function, it will
always show the initial value (here 0), while the user would
expect the last value (here 8).
This logic was introduced for showing the entry-values of
function arguments if they are available, but for some
reason this was added for non-entry-values as well.

One of the tests of amd64-entry-value.exp shows the same
problem for function arguments:
```
s1=s1@entry=11, s2=s2@entry=12, ..., d9=d9@entry=11.5, da=da@entry=12.5
```

I've fixed this by only using the initial values when
explicitely looking for entry values.

Now the local variable is as expected:
```
(gdb) p i
$1 = 8
```

And the test of amd64-entry-value.exp shows the expected
current and entry values of the function arguments:
```
s1=3, s1@entry=11, s2=4, s2@entry=12, ..., d9=3.5, d9@entry=11.5, da=4.5, da@entry=12.5
```

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28987
---
 gdb/dwarf2/loc.c                             | 7 ++++---
 gdb/dwarf2/loc.h                             | 3 ++-
 gdb/testsuite/gdb.arch/amd64-entry-value.exp | 2 +-
 3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index 5b2d58ab44e..c15221eb7a2 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -363,7 +363,8 @@ decode_debug_loc_dwo_addresses (dwarf2_per_cu_data *per_cu,
 
 const gdb_byte *
 dwarf2_find_location_expression (const dwarf2_loclist_baton *baton,
-				 size_t *locexpr_length, const CORE_ADDR pc)
+				 size_t *locexpr_length, const CORE_ADDR pc,
+				 bool at_entry)
 {
   dwarf2_per_objfile *per_objfile = baton->per_objfile;
   struct objfile *objfile = per_objfile->objfile;
@@ -456,7 +457,7 @@ dwarf2_find_location_expression (const dwarf2_loclist_baton *baton,
 	  loc_ptr += bytes_read;
 	}
 
-      if (low == high && unrel_pc == low)
+      if (low == high && unrel_pc == low && at_entry)
 	{
 	  /* This is entry PC record present only at entry point
 	     of a function.  Verify it is really the function entry point.  */
@@ -3920,7 +3921,7 @@ loclist_read_variable_at_entry (struct symbol *symbol, frame_info_ptr frame)
   if (frame == NULL || !get_frame_func_if_available (frame, &pc))
     return value::allocate_optimized_out (symbol->type ());
 
-  data = dwarf2_find_location_expression (dlbaton, &size, pc);
+  data = dwarf2_find_location_expression (dlbaton, &size, pc, true);
   if (data == NULL)
     return value::allocate_optimized_out (symbol->type ());
 
diff --git a/gdb/dwarf2/loc.h b/gdb/dwarf2/loc.h
index 5cf824d3ae2..94e1fbe517e 100644
--- a/gdb/dwarf2/loc.h
+++ b/gdb/dwarf2/loc.h
@@ -39,7 +39,8 @@ extern unsigned int entry_values_debug;
 const gdb_byte *dwarf2_find_location_expression
   (const dwarf2_loclist_baton *baton,
    size_t *locexpr_length,
-   CORE_ADDR pc);
+   CORE_ADDR pc,
+   bool at_entry = false);
 
 /* Find the frame base information for FRAMEFUNC at PC.  START is an
    out parameter which is set to point to the DWARF expression to
diff --git a/gdb/testsuite/gdb.arch/amd64-entry-value.exp b/gdb/testsuite/gdb.arch/amd64-entry-value.exp
index 3c666acc117..c7fea226df7 100644
--- a/gdb/testsuite/gdb.arch/amd64-entry-value.exp
+++ b/gdb/testsuite/gdb.arch/amd64-entry-value.exp
@@ -77,7 +77,7 @@ gdb_continue_to_breakpoint "entry_stack: stacktest"
 
 gdb_test "bt" \
     [multi_line \
-	 "^#0 +stacktest *\\(r1=r1@entry=1, r2=r2@entry=2, \[^\r\n\]+, s1=s1@entry=11, s2=s2@entry=12, \[^\r\n\]+, d9=d9@entry=11\\.5, da=da@entry=12\\.5\\) \[^\r\n\]*" \
+	 "^#0 +stacktest *\\(r1=r1@entry=1, r2=r2@entry=2, \[^\r\n\]+, s1=3, s1@entry=11, s2=4, s2@entry=12, \[^\r\n\]+, d9=3\\.5, d9@entry=11\\.5, da=4\\.5, da@entry=12\\.5\\) \[^\r\n\]*" \
 	 "#1 +0x\[0-9a-f\]+ in main .*"] \
     "entry_stack: bt at entry"
 
-- 
2.35.1