[PATCH] D34263: [preprocessor] When preprocessor option 'SingleFileParseMode' is enabled, parse all directive blocks if the condition uses undefined macros

Argyrios Kyrtzidis via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Jun 19 09:39:18 PDT 2017

akyrtzi added a comment.

In https://reviews.llvm.org/D34263#783770, @voskresensky.vladimir wrote:

> What I find problematic in this patch is PPOpts->SingleFileParseMode checks.
>  Let's suppose we implement what I mentioned above => how is it going to co-exist nicely? I think code will be less understandable with both: check of flag and call to PPCallbacks.
>  What I propose is to move the logic from the PPOpts single flag into PPCallbacks. And from check of flag to query of PPCallbacks.
>  Of course when you create PPCallbacks you can consult global SingleFileParseMode to create default implementation to answer "any symbol is defined", so you get your use-case easily handled, but it also gives others more flexibility.

There is a bit of misunderstanding on what this patch is doing, the patch is not about answering "any symbol is defined". See my cfe-dev email (http://lists.llvm.org/pipermail/cfe-dev/2017-June/054254.html) that provides some more context.
The patch is doing these 2 things:

- Keep track of whether a preprocessor directive condition used a symbol that was not resolved at all
- If above is true then make the preprocessor parse all directive blocks, not arbitrary choose one of them.

Here's an example to clarify the difference:

Say that you have this code:

  @interface A @end
  @interface B @end

1. Without any changes this will happen:
  - TARGET_IOS is unresolved so it will be treated as `0`
  - `#if` block will be skipped, `#else` will be parsed
  - AST will contain `@interface B`

2. With adding a PPCallback that answers "any symbol is defined" (what you are proposing):
  - TARGET_IOS is undefined but the PPCallback sets it to `1`
  - `#if` block will be parsed, `#else` will be skipped
  - AST will contain `@interface A`

3. With this patch and enabling `SingleFileParseMode`
  - TARGET_IOS is unresolved
  - Both `#if` and `#else` blocks will be parsed
  - AST will contain both `@interface A` and `@interface B`

As you can see 2. and 3. above are not overlapping so they can "co-exist nicely" like this:

- TARGET_IOS is undefined, ask the PPCallback if it has a value for it or not
  - If the callback provides a value then parse as 1. or 2. depending on the value
  - If the callback cannot provide a specific value then parse as 3.


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