bpo-31113: Get rid of recursion in the compiler for normal control flow. #3015
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Lib/test/test_compile.py
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| @@ -670,6 +670,11 @@ def __fspath__(self): | |||
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| compile("42", PathLike("test_compile_pathlike"), "single") | |||
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| def test_dfs_next(self): | |||
| # Issue #31113: Stack overflow when compile a long sequence of | |||
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Please use the new "bpo-xxx" format.
Python/compile.c
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| for (j = nblocks; b && !b->b_seen; b = b->b_next) { | ||
| b->b_seen = 1; | ||
| assert(a->a_nblocks < j); | ||
| a->a_postorder[--j] = b; |
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This only works because blocks are emitted in topological order, right? It would be nice to add a comment.
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Or are you using a_postorder as a stack or worklist?
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Yes, since the number of blocks is limited, the space at the end of a_postorder can be used as a stack.
Python/compile.c
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| while (j < nblocks) { | ||
| b = a->a_postorder[j++]; | ||
| for (i = 0; i < b->b_iused; i++) { | ||
| struct instr *instr = instr = &b->b_instr[i]; |
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I don't understand the struct instr *instr = instr part.
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Good catch! This is just a copying error.
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It's telling that you're still getting a stack overflow on Windows :-) |
| dfs(c, instr->i_target, a); | ||
| int i, j; | ||
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| /* Get rid of recursion for normal control flow. |
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Perhaps it is possible to get rid of recursion entirely -- perhaps by allocating a separate stack? Or is it unlikely to have a stack overflow though i_target?
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Perhaps, though this likely will complicate the code. I don't have an example that overflows a stack though i_target. After getting such example I'll try to get rid of recursion entirely.
Python/compile.c
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| int i; | ||
| basicblock *next; | ||
| b = sp[-1]; | ||
| if (b->b_seen == 2) { |
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Perhaps add some comment as to the different values of b_seen? Does 1 mean "queued for processing" and 2 "already processed"?
Python/compile.c
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| if (instr->i_jrel || instr->i_jabs) | ||
| dfs(c, instr->i_target, a, j); | ||
| } | ||
| a->a_postorder[a->a_nblocks++] = b; |
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Can we add something like assert(a->a_nblocks <= j) here? The stack trick is smart but I'd like to make sure it's not buggy.
Python/compile.c
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| while (sp != stack) { | ||
| int i; | ||
| basicblock *next; | ||
| b = sp[-1]; |
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Why don't we decrement sp here instead of setting b->b_seen = 2 below?
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This is closer to the current implementation. If just pop a block from the stack and keep b->b_seen == 1, all seems working, but the result is different. I don't know what result is more correct, but larger depth is safer. Clearing b->b_seen after popping a block causes an infinite loop. Current code contains bugs (see bpo-24340), may be after fixing them this code could be simplified.
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What is the status of this? |
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I'm wary of backporting this and would prefer it to be 3.7-only. |
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I think a good experiment would be to take the PR for bpo-17611 and make it non-recursive. It should be similar to this PR in principle. |
https://bugs.python.org/issue31113