[PATCH] D41880: Adding nocf_check attribute for cf-protection fine tuning

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


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Comment at: include/clang/CodeGen/CGFunctionInfo.h:519
+  /// Whether this function has nocf_check attribute.
+  unsigned NoCfCheck : 1;
+
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oren_ben_simhon wrote:
> aaron.ballman wrote:
> > This is unfortunate -- it bumps the bit-field over 32 bits. Can the bit be stolen from elsewhere?
> The field is orthogonal to the other fields moreover i think that double meaning of the same field will lead to future bugs. The class is not a compact packed structure, so i don't feel it worth the confusion.
The class packs its fields for space savings because it's used fairly frequently; if we can keep the size from having to use another allocation unit, that's better. I wasn't suggesting we make a bit have double meaning, I was wondering if we could reduce the size of one of the other fields. For instance, I'd be surprised if we need all 8 bits for calling conventions, so we might be able to reduce that to a 7-bit field.


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Comment at: lib/Sema/SemaDeclAttr.cpp:1979-1980
+static void handleNoCfCheckAttr(Sema &S, Decl *D, const AttributeList &Attrs) {
+  if (!S.getLangOpts().CFProtectionBranch)
+    S.Diag(Attrs.getLoc(), diag::warn_nocf_check_attribute_ignored);
+  else
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oren_ben_simhon wrote:
> aaron.ballman wrote:
> > Can you use the `LangOpts` field to specify this in Attr.td? Then you can go back to the simple handler.
> When using LangOpts field in Attr.td, the diagnostic warning will not be descriptive as i use here (use -fcf-protection flag...).
That's true, and this code is fine for now. However, it does suggest that the declarative handler could be improved to support this sort of thing -- the same issue is present with *all* attributes gated on a language option.


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Comment at: test/Sema/attr-nocf_check.cpp:1
+// RUN: %clang_cc1 -verify -fcf-protection=branch -target-feature +ibt -fsyntax-only %s
+
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oren_ben_simhon wrote:
> aaron.ballman wrote:
> > aaron.ballman wrote:
> > > For better test coverage, you can switch to the `[[gnu::nocf_check]]` spelling in this file and pass `-std=c++11`
> > This one also likely needs an explicit triple.
> Since it doesn't test the functionality of this specific attribute, I believe it is an overkill to switch to [[gnu::nocf_check]] spelling.
> Since it doesn't test the functionality of this specific attribute, I believe it is an overkill to switch to [[gnu::nocf_check]] spelling.

C++ attribute spellings have slightly different code paths than GNU attribute spellings, so the test isn't overkill. Also, this is a C++ test file that already uses C++11, so adding the C++11 spelling is an improvement over using the GNU spelling (that's already covered with C tests).

It's not critical, but it's still a strict improvement.


Repository:
  rL LLVM

https://reviews.llvm.org/D41880