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

[Guinevere Larsen <blarsen@redhat.com>]

On 07/12/2023 13:47, Hannes Domani wrote:
> 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
> ---
I'm not sure what's going on in this code, but I tested this on fedora 
37 with both gcc and clang and this patch fixes the issue without adding 
regressions.

Tested-By: Guinevere Larsen <blarsen@redhat.com>

-- 
Cheers,
Guinevere Larsen
She/Her/Hers

>   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"
>