YJIT: Fix raw sample stack lengths in exit traces (#7728)

yjit-trace-exits appends a synthetic sample for the instruction being
exited, but we didn't increment the size of the stack. Fixing this count
correctly lets us successfully generate a flamegraph from the exits.

I also replaced the line number for instructions with 0, as I don't
think the previous value had meaning.

Co-authored-by: Adam Hess <HParker@github.com>

yjit.c

@@ -186,8 +186,9 @@ rb_yjit_exit_locations_dict(VALUE *yjit_raw_samples, int *yjit_line_samples, int
         int line_num = (int)yjit_line_samples[idx];
         idx++;
 
-        rb_ary_push(raw_samples, SIZET2NUM(num));
-        rb_ary_push(line_samples, INT2NUM(line_num));
+        // + 1 as we append an additional sample for the insn
+        rb_ary_push(raw_samples, SIZET2NUM(num + 1));
+        rb_ary_push(line_samples, INT2NUM(line_num + 1));
 
         // Loop through the length of samples_len and add data to the
         // frames hash. Also push the current value onto the raw_samples

yjit.rb

@@ -67,7 +67,7 @@ def self.exit_locations
     # [ length, line_1, line_2, line_n, ..., dummy value, count
     i = 0
     while i < raw_samples.length
-      stack_length = raw_samples[i] + 1
+      stack_length = raw_samples[i]
       i += 1 # consume the stack length
 
       sample_count = raw_samples[i + stack_length]

yjit/src/stats.rs

@@ -724,10 +724,8 @@ pub extern "C" fn rb_yjit_record_exit_stack(exit_pc: *const VALUE)
         // Push the insn value into the yjit_raw_samples Vec.
         yjit_raw_samples.push(VALUE(insn as usize));
 
-        // Push the current line onto the yjit_line_samples Vec. This
-        // points to the line in insns.def.
-        let line = yjit_line_samples.len() - 1;
-        yjit_line_samples.push(line as i32);
+        // We don't know the line
+        yjit_line_samples.push(0);
 
         // Push number of times seen onto the stack, which is 1
         // because it's the first time we've seen it.