Micro-optimize list index range checks by rhettinger · Pull Request #9784 · python/cpython (original) (raw)
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Old code
_list_item:
testq %rsi, %rsi
js L282
cmpq %rsi, 16(%rdi)
jg L283
...
L283:
movq 24(%rdi), %rax
movq (%rax,%rsi,8), %rax
addq $1, (%rax)
ret
New code
_list_item:
cmpq 16(%rdi), %rsi
jb L282
...
L282:
movq 24(%rdi), %rax
movq (%rax,%rsi,8), %rax
addq $1, (%rax)
ret
It would be interesting to run Linux perf on some representative examples to understand how the function call is affecting cache misses, references and branch predictions. (See #6493 as an example).
| optimization manual found at: |
|---|
| https://www.agner.org/optimize/optimizing\_cpp.pdf |
| */ |
| return (size_t) i < (size_t) limit; |
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I'm not sure that the behaviour is well defined in C. I fear that it's Undefined Behaviour. @benjaminp @gpshead: What do you think ?
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If it's well defined, why should we hack such micro optimization? Why compilers would not implement the optimization themself?
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I think because they don't know that Py_SIZE(op) is non-negative.
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It is well defined. It is used for example in the STL implementations.
But there was not found any difference in microbenchmark results on 64-bit platforms in previous discussion in bpo-28397.
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Regardless of if this change is measurable, i like the way the code looks afterwards, getting rid of the repeated verbose i < 0 || i >= Py_SIZE(spam) everywhere. so +1 from me.
| @@ -208,6 +208,19 @@ PyList_Size(PyObject *op) |
|---|
| return Py_SIZE(op); |
| } |
| static inline int |
| valid_index(Py_ssize_t i, Py_ssize_t limit) |
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Why not just define this as taking two size_t parameters instead of doing the casting below. The casts then happen implicitly at all call sites.
This was referenced
Jun 6, 2024
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