Summary:
Previously, we considered a function which modifies its parameters to be impure even though it might not be modifying the underlying value. This resulted in FPs like the following program in Java:
```
void fresh_pure(int[] a) {
a = new int[1];
}
```
Similarly, in C++, we considered the following program as impure because it was writing to `s`:
```
Simple* reassign_pure(Simple* s) {
s = new Simple{2};
return s;
}
```
This diff fixes that issue by starting the check for address equivalnce in pre-post not directly from the addresses of the stack variables, but from the addresses pointed to by these stack variables. That means, we only consider things to be impure if the actual values pointed by the parameters change.
Reviewed By: skcho
Differential Revision: D18113846
fbshipit-source-id: 3d7c712f3
Summary: If we have no pulse summary (most likely caused by pulse finding a legit issue with the code), let's consider the function as impure.
Reviewed By: jvillard
Differential Revision: D17906016
fbshipit-source-id: 671d3e0ba
Summary:
Introduce a new experimental checker (`--impurity`) that detects
impurity information, tracking which parameters and global variables
of a function are modified. The checker relies on Pulse to detect how
the state changes: it traverses the pre and post pairs starting from
the parameter/global variable and finds where the pre and post heaps
diverge. At diversion points, we expect to see WrittenTo/Invalid attributes
containing a trace of how the address was modified. We use these to
construct the trace of impurity.
This checker is a complement to the purity checker that exists mainly
for Java (and used for cost and loop-hoisting analyses). The aim of
this new experimental checker is to rely on Pulse's precise
memory treatment and come up with a more precise im(purity)
analysis. To distinguish the two checkers, we introduce a new issue
type `IMPURE_FUNCTION` that reports when a function is impure, rather
than when it is pure (as in the purity checker).
TODO:
- improve the analysis to rely on impurity information of external
library calls. Currently, all library calls are assumed to be nops,
hence pure.
- de-entangle Pulse reporting from analysis.
Reviewed By: skcho
Differential Revision: D17051567
fbshipit-source-id: 5e10afb4f
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