[PATCH v4 3/8] Add support for non-contiguous blocks to find_pc_partial_function

Kevin Buettner
2018-08-22T17:11:23Z

This change adds an optional output parameter BLOCK to
find_pc_partial_function. If BLOCK is non-null, then *BLOCK will be
set to the address of the block corresponding to the function symbol
if such a symbol was found during lookup. Otherwise it's set to the
NULL value. Callers may wish to use the block information to
determine whether the block contains any non-contiguous ranges. The
caller may also iterate over or examine those ranges.

When I first started looking at the broken stepping behavior associated
with functions w/ non-contiguous ranges, I found that I could "fix"
the problem by disabling the find_pc_partial_function cache. It would
sometimes happen that the PC passed in would be between the low and
high cache values, but would be in some other function that happens to
be placed in between the ranges for the cached function. This caused
incorrect values to be returned.

So dealing with this cache turns out to be very important for fixing
this problem. I explored three different ways of dealing with the
cache.

My first approach was to clear the cache when a block was encountered
with more than one range. This would cause the non-cache pathway to
be executed on the next call to find_pc_partial_function.

Another approach, which I suspect is slightly faster, checks to see
whether the PC is within one of the ranges associated with the cached
block. If so, then the cached values can be used. It falls back to
the original behavior if there is no cached block.

The current approach, suggested by Simon Marchi, is to restrict the
low/high pc values recorded for the cache to the beginning and end of
the range containing the PC value under consideration. This allows us
to retain the simple (and fast) test for determining whether the
memoized (cached) values apply to the PC passed to
find_pc_partial_function.

Another choice that had to be made regards setting *ADDRESS and
*ENDADDR. There are three possibilities which might make sense:

1) *ADDRESS and *ENDADDR represent the lowest and highest address
of the function.
2) *ADDRESS and *ENDADDR are set to the start and end address of
the range containing the entry pc.
3) *ADDRESS and *ENDADDR are set to the start and end address of
the range in which PC is found.

An earlier version of this patch implemented option #1. I found out
that it's not very useful though and, in fact, returns results that
are incorrect when used in the context of determining the start and
end of the function for doing prologue analysis. While debugging a
function in which the entry pc was in the second range (of a function
containing two non-contiguous ranges), I noticed that
amd64_skip_prologue called find_pc_partial_function - the returned
start address was set to the beginning of the first range. This is
incorrect for this function. What was also interesting was that this
first invocation of find_pc_partial_function correctly set the cache
for the PC on which it had been invoked, but a slightly later call
from skip_prologue_using_sal could not use this cached value because
it was now being used to lookup the very lowest address of the
function - which is in a range not containing the entry pc.

Option #2 is attractive as it would provide a desirable result
when used in the context of prologue analysis. However, many callers,
including some which do prologue analysis want the condition
*ADDRESS <= PC < *ENDADDR to hold. This will not be the case when
find_pc_partial_function is called on a PC that's in a non-entry-pc
range. A later patch to this series adds
find_function_entry_range_from_pc as a wrapper of
find_pc_partial_function.

Option #3 causes the *ADDRESS <= PC < *ENDADDR property to hold. If
find_pc_partial_function is called with a PC that's within entry pc's
range, then it will correctly return the limits of that range. So, if
the result of a minsym search is passed to find_pc_partial_function
to find the limits, then correct results will be achieved. Returned
limits (for prologue analysis) won't be correct when PC is within some
other (non-entry-pc) range. I don't yet know how big of a problem
this might be; I'm guessing that it won't be a serious problem - if a
compiler generates functions which have non-contiguous ranges, then it
also probably generates DWARF2 CFI which makes a lot of the old
prologue analysis moot.

I've implemented option #3 for this version of the patch. I don't see
any regressions for x86-64. Moreover, I don't expect to see
regressions for other targets either simply because
find_pc_partial_function behaves the same as it did before for the
contiguous address range case. That said, there may be some
adjustments needed if GDB encounters a function requiring prologue
analysis which occupies non-contiguous ranges.

gdb/ChangeLog:

* symtab.h (find_pc_partial_function): Add new parameter `block'.
* blockframe.c (cache_pc_function_block): New static global.
(clear_pc_function_cache): Clear cache_pc_function_block.
(find_pc_partial_function): Move comment to symtab.h. Add
support for non-contiguous blocks.
---
gdb/blockframe.c | 99 +++++++++++++++++++++++++++++++++++++++++++++-----------
gdb/symtab.h | 40 ++++++++++++++++++++---
2 files changed, 116 insertions(+), 23 deletions(-)

diff --git a/gdb/blockframe.c b/gdb/blockframe.c
index 6a018cc..6dc736e 100644
--- a/gdb/blockframe.c
+++ b/gdb/blockframe.c
@@ -165,13 +165,35 @@ find_pc_sect_containing_function (CORE_ADDR pc, struct obj_section *section)
return block_containing_function (bl);
}

-/* These variables are used to cache the most recent result
- of find_pc_partial_function. */
+/* These variables are used to cache the most recent result of
+ find_pc_partial_function.
+
+ The addresses cache_pc_function_low and cache_pc_function_high
+ record the range in which PC was found during the most recent
+ successful lookup. When the function occupies a single contiguous
+ address range, these values correspond to the low and high
+ addresses of the function. (The high address is actually one byte
+ beyond the last byte of the function.) For a function with more
+ than one (non-contiguous) range, the range in which PC was found is
+ used to set the cache bounds.
+
+ When determining whether or not these cached values apply to a
+ particular PC value, PC must be within the range specified by
+ cache_pc_function_low and cache_pc_function_high. In addition to
+ PC being in that range, cache_pc_section must also match PC's
+ section. See find_pc_partial_function() for details on both the
+ comparison as well as how PC's section is determined.
+
+ The other values aren't used for determining whether the cache
+ applies, but are used for setting the outputs from
+ find_pc_partial_function. cache_pc_function_low and
+ cache_pc_function_high are used to set outputs as well. */

static CORE_ADDR cache_pc_function_low = 0;
static CORE_ADDR cache_pc_function_high = 0;
static const char *cache_pc_function_name = 0;
static struct obj_section *cache_pc_function_section = NULL;
+static const struct block *cache_pc_function_block = nullptr;

/* Clear cache, e.g. when symbol table is discarded. */

@@ -182,24 +204,14 @@ clear_pc_function_cache (void)
cache_pc_function_high = 0;
cache_pc_function_name = (char *) 0;
cache_pc_function_section = NULL;
+ cache_pc_function_block = nullptr;
}

-/* Finds the "function" (text symbol) that is smaller than PC but
- greatest of all of the potential text symbols in SECTION. Sets
- *NAME and/or *ADDRESS conditionally if that pointer is non-null.
- If ENDADDR is non-null, then set *ENDADDR to be the end of the
- function (exclusive), but passing ENDADDR as non-null means that
- the function might cause symbols to be read. This function either
- succeeds or fails (not halfway succeeds). If it succeeds, it sets
- *NAME, *ADDRESS, and *ENDADDR to real information and returns 1.
- If it fails, it sets *NAME, *ADDRESS and *ENDADDR to zero and
- returns 0. */
-
-/* Backward compatibility, no section argument. */
+/* See symtab.h. */

int
find_pc_partial_function (CORE_ADDR pc, const char **name, CORE_ADDR *address,
- CORE_ADDR *endaddr)
+ CORE_ADDR *endaddr, const struct block **block)
{
struct obj_section *section;
struct symbol *f;
@@ -241,17 +253,62 @@ find_pc_partial_function (CORE_ADDR pc, const char **name, CORE_ADDR *address,
if (compunit_symtab != NULL)
{
/* Checking whether the msymbol has a larger value is for the
- "pathological" case mentioned in print_frame_info. */
+ "pathological" case mentioned in stack.c:find_frame_funname.
+
+ We use BLOCK_ENTRY_PC instead of BLOCK_START_PC for this
+ comparison because the minimal symbol should refer to the
+ function's entry pc which is not necessarily the lowest
+ address of the function. This will happen when the function
+ has more than one range and the entry pc is not within the
+ lowest range of addresses. */
f = find_pc_sect_function (mapped_pc, section);
if (f != NULL
&& (msymbol.minsym == NULL
- || (BLOCK_START (SYMBOL_BLOCK_VALUE (f))
+ || (BLOCK_ENTRY_PC (SYMBOL_BLOCK_VALUE (f))
>= BMSYMBOL_VALUE_ADDRESS (msymbol))))
{
- cache_pc_function_low = BLOCK_START (SYMBOL_BLOCK_VALUE (f));
- cache_pc_function_high = BLOCK_END (SYMBOL_BLOCK_VALUE (f));
+ const struct block *b = SYMBOL_BLOCK_VALUE (f);
+
cache_pc_function_name = SYMBOL_LINKAGE_NAME (f);
cache_pc_function_section = section;
+ cache_pc_function_block = b;
+
+ /* For blocks occupying contiguous addresses (i.e. no gaps),
+ the low and high cache addresses are simply the start
+ and end of the block.
+
+ For blocks with non-contiguous ranges, we have to search
+ for the range containing mapped_pc and then use the start
+ and end of that range.
+
+ This causes the returned *ADDRESS and *ENDADDR values to
+ be limited to the range in which mapped_pc is found. See
+ comment preceding declaration of find_pc_partial_function
+ in symtab.h for more information. */
+
+ if (BLOCK_CONTIGUOUS_P (b))
+ {
+ cache_pc_function_low = BLOCK_START (b);
+ cache_pc_function_high = BLOCK_END (b);
+ }
+ else
+ {
+ int i;
+ for (i = 0; i < BLOCK_NRANGES (b); i++)
+ {
+ if (BLOCK_RANGE_START (b, i) <= mapped_pc
+ && mapped_pc < BLOCK_RANGE_END (b, i))
+ {
+ cache_pc_function_low = BLOCK_RANGE_START (b, i);
+ cache_pc_function_high = BLOCK_RANGE_END (b, i);
+ break;
+ }
+ }
+ /* Above loop should exit via the break. */
+ gdb_assert (i < BLOCK_NRANGES (b));
+ }
+
+
goto return_cached_value;
}
}
@@ -281,6 +338,7 @@ find_pc_partial_function (CORE_ADDR pc, const char **name, CORE_ADDR *address,
cache_pc_function_name = MSYMBOL_LINKAGE_NAME (msymbol.minsym);
cache_pc_function_section = section;
cache_pc_function_high = minimal_symbol_upper_bound (msymbol);
+ cache_pc_function_block = nullptr;

return_cached_value:

@@ -311,6 +369,9 @@ find_pc_partial_function (CORE_ADDR pc, const char **name, CORE_ADDR *address,
*endaddr = cache_pc_function_high;
}

+ if (block != nullptr)
+ *block = cache_pc_function_block;
+
return 1;
}

diff --git a/gdb/symtab.h b/gdb/symtab.h
index 0b155d0..746b5e6 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -1694,10 +1694,42 @@ extern struct symbol *find_pc_sect_containing_function

extern struct symbol *find_symbol_at_address (CORE_ADDR);

-/* lookup function from address, return name, start addr and end addr. */
-
-extern int find_pc_partial_function (CORE_ADDR, const char **, CORE_ADDR *,
- CORE_ADDR *);
+/* Finds the "function" (text symbol) that is smaller than PC but
+ greatest of all of the potential text symbols in SECTION. Sets
+ *NAME and/or *ADDRESS conditionally if that pointer is non-null.
+ If ENDADDR is non-null, then set *ENDADDR to be the end of the
+ function (exclusive). If the optional parameter BLOCK is non-null,
+ then set *BLOCK to the address of the block corresponding to the
+ function symbol, if such a symbol could be found during the lookup;
+ nullptr is used as a return value for *BLOCK if no block is found.
+ This function either succeeds or fails (not halfway succeeds). If
+ it succeeds, it sets *NAME, *ADDRESS, and *ENDADDR to real
+ information and returns 1. If it fails, it sets *NAME, *ADDRESS
+ and *ENDADDR to zero and returns 0.
+
+ If the function in question occupies non-contiguous ranges,
+ *ADDRESS and *ENDADDR are (subject to the conditions noted above) set
+ to the start and end of the range in which PC is found. Thus
+ *ADDRESS <= PC < *ENDADDR with no intervening gaps (in which ranges
+ from other functions might be found).
+
+ This property allows find_pc_partial_function to be used (as it had
+ been prior to the introduction of non-contiguous range support) by
+ various tdep files for finding a start address and limit address
+ for prologue analysis. This still isn't ideal, however, because we
+ probably shouldn't be doing prologue analysis (in which
+ instructions are scanned to determine frame size and stack layout)
+ for any range that doesn't contain the entry pc. Moreover, a good
+ argument can be made that prologue analysis ought to be performed
+ starting from the entry pc even when PC is within some other range.
+ This might suggest that *ADDRESS and *ENDADDR ought to be set to the
+ limits of the entry pc range, but that will cause the
+ *ADDRESS <= PC < *ENDADDR condition to be violated; many of the
+ callers of find_pc_partial_function expect this condition to hold. */
+
+extern int find_pc_partial_function (CORE_ADDR pc, const char **name,
+ CORE_ADDR *address, CORE_ADDR *endaddr,
+ const struct block **block = nullptr);

/* Return the type of a function with its first instruction exactly at
the PC address. Return NULL otherwise. */