Guard out-of-range shift counts in integer opcodes#559
Open
uwezkhan wants to merge 1 commit into
Open
Conversation
Author
|
gentle ping |
Contributor
There was a problem hiding this comment.
Pull request overview
This PR makes integer shift opcodes deterministic and NumPy-compatible when the per-element shift count is negative or exceeds the operand bit width, avoiding undefined behavior in the C++ interpreter loop.
Changes:
- Guard
OP_LSHIFT_*to return0when the shift count is out-of-range. - Guard
OP_RSHIFT_*to clamp out-of-range counts towidth-1to preserve sign-fill behavior. - Add a regression test covering out-of-range shift counts for
i4andi8.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
numexpr/interp_body.cpp |
Adds per-element range checks for 32-bit and 64-bit integer shift opcodes to avoid UB and match NumPy semantics. |
numexpr/tests/test_numexpr.py |
Adds a regression test asserting NumExpr matches NumPy for out-of-range shift counts. |
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
Comment on lines
+464
to
+469
| for dtype in ('i4', 'i8'): | ||
| x = array([5, -5, 0], dtype=dtype) | ||
| for count in (-1, 64, 200): | ||
| y = array([count] * len(x), dtype=dtype) | ||
| assert_array_equal(evaluate("x << y"), x << y) | ||
| assert_array_equal(evaluate("x >> y"), x >> y) |
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
The integer shift opcodes hand the per-element shift count straight to C++
<</>>. Whena<<bora>>bruns with a count that is negative or at least the operand width (32 for int, 64 for long long) the shift is undefined behavior. On arm64 `evaluate("a<<b")" with b=100 returns 80 because the hardware masks the count, while NumPy returns 0, and a UBSAN build traps right at the shift. Both int and long long, left and right, hit this.After the change an out-of-range left shift yields 0 and an out-of-range right shift clamps the count to width-1 so the sign bit fills, which is the result NumPy gives. The guard sits in the opcode next to the existing div/mod guards because the count is only known per element at run time, so a caller-side check could not cover array shift counts. Tradeoff is one extra unsigned compare per element; in-range counts keep the same value and the branch is well predicted.