[PATCH] D126676: [clang] Disallow differences in defines used for creating and using PCH

[Martin Storsjö via Phabricator via cfe-commits]

mstorsjo created this revision.
mstorsjo added reviewers: aaron.ballman, rnk.
Herald added a project: All.
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Some amount of differences between defines when creating and using
a PCH were intentionally allowed in
c379c072405f39bca1d3552408fc0427328e8b6d (and later extended in
b63687519610a73dd565be1fec28332211b4df5b).

When using a PCH (or when picking a PCH out of a directory containing
multiple candidates) Clang used to accept the header if there were
defines on the command line when creating the PCH that are missing
when using the PCH, or vice versa, defines only set when using the
PCH.

The only cases where Clang explicitly rejected the use of a PCH
is if there was an explicit conflict between the options, e.g.
-DFOO=1 vs -DFOO=2, or -DFOO vs -UFOO.

The latter commit added a FIXME that we really should check whether
mismatched defines actually were used somewhere in the PCH, so that
the define would affect the outcome. This FIXME has stood unaddressed
since 2012.

This differs from GCC, which rejects PCH files if the defines differ
at all.

This is also problematic when using a directory containing multiple
candidate PCH files, one built with -DFOO and one without. When
attempting to include a PCH and iterating over the candidates in the
directory, Clang will essentially pick the first one out of the two,
even if there existed a better, exact match in the directory.

This fixes https://github.com/lhmouse/mcfgthread/issues/63.

This is likely to be controversial. In tree, 84 tests break by
making this change. This patch only fixes up the core tests for
this feature (to bring it up for discussion); out of the rest, some
probably are easy to fix, while others seem to be designed to specifically
test cases with such discrepancies.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D126676

Files:
  clang/lib/Serialization/ASTReader.cpp
  clang/test/PCH/fuzzy-pch.c
  clang/test/PCH/pch-dir.c

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