Mergeback rc/1.20 to master#1217
Merged
aschackmull merged 4 commits intogithub:masterfrom Apr 8, 2019
Merged
Conversation
This commit changes how data flow works in the following code.
MyType x = source();
defineByReference(&x);
sink(x);
The question here is whether there should be flow from `source` to
`sink`. Such flow is desirable if `defineByReference` doesn't write to
all of `x`, but it's undesirable if `defineByReference` is a typical
init function in `C` that writes to every field or if
`defineByReference` is `memcpy` or `memset` on the full range.
Before 1.20.0, there would be flow from `source` to `sink` in case `x`
happened to be modeled with `BlockVar` but not in case `x` happened to
be modelled with SSA. The choice of modelling depends on an analysis of
how `x` is used elsewhere in the function, and it's supposed to be an
internal implementation detail that there are two ways to model
variables. In 1.20.0, I changed the `BlockVar` behavior so it worked the
same as SSA, never allowing that flow. It turns out that this change
broke a customer's query.
This commit reverts `BlockVar` to its old behavior of letting flow
propagate past the `defineByReference` call and then regains consistency
by changing all variables that are ever defined by reference to be
modelled with `BlockVar` instead of SSA. This means we now get too much
flow in certain cases, but that appears to be better overall than
getting too little flow. See also the discussion in CPP-336.
This query uses data flow for nullness analysis, which is always going to be a large overapproximation. The overapproximation became too big for one of the test cases after the recent change to make data flow go across assignment by reference. To make this query more conservative, it will now only report that the `pDacl` argument can be null if there isn't also evidence that it can be non-null.
C++: Let data flow past definition by reference
aschackmull
approved these changes
Apr 8, 2019
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
3 participants
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.
No description provided.