Applied in changeset git|1a05dc03f953830564c272665c47a61e53550f3e.
Make backtrace generation work outward from current frame
This fixes multiple bugs found in the partial backtrace
optimization added in 3b24b7914c16930bfadc89d6aff6326a51c54295.
These bugs occurs when passing a start argument to caller where
the start argument lands on a iseq frame without a pc.
Before this commit, the following code results in the same
line being printed twice, both for the #each method.
def a; [1].group_by { b } end
def b; puts(caller(2, 1).first, caller(3, 1).first) end
a
After this commit and in Ruby 2.7, the lines are different,
with the first line being for each and the second for group_by.
Before this commit, the following code can either segfault or
result in an infinite loop:
def foo
caller_locations(2, 1).inspect # segfault
caller_locations(2, 1)[0].path # infinite loop
end
1.times.map { 1.times.map { foo } }
After this commit, this code works correctly.
This commit completely refactors the backtrace handling.
Instead of processing the backtrace from the outermost
frame working in, process it from the innermost frame
working out. This is much faster for partial backtraces,
since you only access the control frames you need to in
order to construct the backtrace.
To handle cfunc frames in the new design, they start
out with no location information. We increment a counter
for each cfunc frame added. When an iseq frame with pc
is accessed, after adding the iseq backtrace location,
we use the location for the iseq backtrace location for
all of the directly preceding cfunc backtrace locations.
If the last backtrace line is a cfunc frame, we continue
scanning for iseq frames until the end control frame, and
use the location information from the first one for the
trailing cfunc frames in the backtrace.
As only rb_ec_partial_backtrace_object uses the new
backtrace implementation, remove all of the function
pointers and inline the functions. This makes the
process easier to understand.
Restore the Ruby 2.7 implementation of backtrace_each and
use it for all the other functions that called
backtrace_each other than rb_ec_partial_backtrace_object.
All other cases requested the entire backtrace, so there
is no advantage of using the new algorithm for those.
Additionally, there are implicit assumptions in the other
code that the backtrace processing works inward instead
of outward.
Remove the cfunc/iseq union in rb_backtrace_location_t,
and remove the prev_loc member for cfunc. Both cfunc and
iseq types can now have iseq and pc entries, so the
location information can be accessed the same way for each.
This avoids the need for a extra backtrace location entry
to store an iseq backtrace location if the final entry in
the backtrace is a cfunc. This is also what fixes the
segfault and infinite loop issues in the above bugs.
Here's Ruby pseudocode for the new algorithm, where start
and length are the arguments to caller or caller_locations:
end_cf = VM.end_control_frame.next
cf = VM.start_control_frame
size = VM.num_control_frames - 2
bt = []
cfunc_counter = 0
if length.nil? || length > size
length = size
end
while cf != end_cf && bt.size != length
if cf.iseq?
if cf.instruction_pointer?
if start > 0
start -= 1
else
bt << cf.iseq_backtrace_entry
cf_counter.times do |i|
bt[-1 - i].loc = cf.loc
end
cfunc_counter = 0
end
end
elsif cf.cfunc?
if start > 0
start -= 1
else
bt << cf.cfunc_backtrace_entry
cfunc_counter += 1
end
end
cf = cf.prev
end
if cfunc_counter > 0
while cf != end_cf
if (cf.iseq? && cf.instruction_pointer?)
cf_counter.times do |i|
bt[-i].loc = cf.loc
end
end
cf = cf.prev
end
end
With the following benchmark, which uses a call depth of
around 100 (common in many Ruby applications):
class T
def test(depth, &block)
if depth == 0
yield self
else
test(depth - 1, &block)
end
end
def array
Array.new
end
def first
caller_locations(1, 1)
end
def full
caller_locations
end
end
t = T.new
t.test((ARGV.first || 100).to_i) do
Benchmark.ips do |x|
x.report ('caller_loc(1, 1)') {t.first}
x.report ('caller_loc') {t.full}
x.report ('Array.new') {t.array}
x.compare!
end
end
Results before commit:
Calculating -------------------------------------
caller_loc(1, 1) 281.159k (_ 0.7%) i/s - 1.426M in 5.073055s
caller_loc 15.836k (_ 2.1%) i/s - 79.450k in 5.019426s
Array.new 1.852M (_ 2.5%) i/s - 9.296M in 5.022511s
Comparison:
Array.new: 1852297.5 i/s
caller_loc(1, 1): 281159.1 i/s - 6.59x (_ 0.00) slower
caller_loc: 15835.9 i/s - 116.97x (_ 0.00) slower
Results after commit:
Calculating -------------------------------------
caller_loc(1, 1) 562.286k (_ 0.8%) i/s - 2.858M in 5.083249s
caller_loc 16.402k (_ 1.0%) i/s - 83.200k in 5.072963s
Array.new 1.853M (_ 0.1%) i/s - 9.278M in 5.007523s
Comparison:
Array.new: 1852776.5 i/s
caller_loc(1, 1): 562285.6 i/s - 3.30x (_ 0.00) slower
caller_loc: 16402.3 i/s - 112.96x (_ 0.00) slower
This shows that the speed of caller_locations(1, 1) has roughly
doubled, and the speed of caller_locations with no arguments
has improved slightly. So this new algorithm is significant faster,
much simpler, and fixes bugs in the previous algorithm.
Fixes [Bug #18053]
Updated by 
Updated by 
Updated by jeremyevans (Jeremy Evans) 
Updated by