mirrors/cpython
1
0
Fork 0
mirror of https://github.com/python/cpython.git synced 2025-08-01 23:53:15 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 8

Issue #6042:

Browse source 
lnotab-based tracing is very complicated and isn't documented very well.  There
were at least 3 comment blocks purporting to document co_lnotab, and none did a
very good job. This patch unifies them into Objects/lnotab_notes.txt which
tries to completely capture the current state of affairs.

I also discovered that we've attached 2 layers of patches to the basic tracing
scheme. The first layer avoids jumping to instructions that don't start a line,
to avoid problems in if statements and while loops.  The second layer
discovered that jumps backward do need to trace at instructions that don't
start a line, so it added extra lnotab entries for 'while' and 'for' loops, and
added a special case for backward jumps within the same line. I replaced these
patches by just treating forward and backward jumps differently.
This commit is contained in:
Jeffrey Yasskin 2009-05-23 23:23:01 +00:00
parent 3724d6c392
commit 655d835415
6 changed files with 157 additions and 209 deletions
Download patch file Download diff file Expand all files Collapse all files

54
Python/compile.c
View file

@ -1646,9 +1646,6 @@ compiler_for(struct compiler *c, stmt_ty s)
VISIT(c, expr, s->v.For.iter);
ADDOP(c, GET_ITER);
compiler_use_next_block(c, start);
/* for expressions must be traced on each iteration,
so we need to set an extra line number. */
c->u->u_lineno_set = false;
ADDOP_JREL(c, FOR_ITER, cleanup);
VISIT(c, expr, s->v.For.target);
VISIT_SEQ(c, stmt, s->v.For.body);
@ -1694,9 +1691,6 @@ compiler_while(struct compiler *c, stmt_ty s)
if (!compiler_push_fblock(c, LOOP, loop))
return 0;
if (constant == -1) {
/* while expressions must be traced on each iteration,
so we need to set an extra line number. */
c->u->u_lineno_set = false;
VISIT(c, expr, s->v.While.test);
ADDOP_JABS(c, POP_JUMP_IF_FALSE, anchor);
}
@ -3493,51 +3487,9 @@ blocksize(basicblock *b)
return size;
}
/* All about a_lnotab.
c_lnotab is an array of unsigned bytes disguised as a Python string.
It is used to map bytecode offsets to source code line #s (when needed
for tracebacks).
The array is conceptually a list of
(bytecode offset increment, line number increment)
pairs. The details are important and delicate, best illustrated by example:
byte code offset source code line number
0 1
6 2
50 7
350 307
361 308
The first trick is that these numbers aren't stored, only the increments
from one row to the next (this doesn't really work, but it's a start):
0, 1, 6, 1, 44, 5, 300, 300, 11, 1
The second trick is that an unsigned byte can't hold negative values, or
values larger than 255, so (a) there's a deep assumption that byte code
offsets and their corresponding line #s both increase monotonically, and (b)
if at least one column jumps by more than 255 from one row to the next, more
than one pair is written to the table. In case #b, there's no way to know
from looking at the table later how many were written. That's the delicate
part. A user of c_lnotab desiring to find the source line number
corresponding to a bytecode address A should do something like this
lineno = addr = 0
for addr_incr, line_incr in c_lnotab:
addr += addr_incr
if addr > A:
return lineno
lineno += line_incr
In order for this to work, when the addr field increments by more than 255,
the line # increment in each pair generated must be 0 until the remaining addr
increment is < 256. So, in the example above, assemble_lnotab (it used
to be called com_set_lineno) should not (as was actually done until 2.2)
expand 300, 300 to 255, 255, 45, 45,
but to 255, 0, 45, 255, 0, 45.
*/
/* Appends a pair to the end of the line number table, a_lnotab, representing
the instruction's bytecode offset and line number. See
Objects/lnotab_notes.txt for the description of the line number table. */
static int
assemble_lnotab(struct assembler *a, struct instr *i)