[CodeGen] [CUDA] Add the ability set default attrs on functions in linked modules.

Summary:
Now when you ask clang to link in a bitcode module, you can tell it to
set attributes on that module's functions to match what we would have
set if we'd emitted those functions ourselves.

This is particularly important for fast-math attributes in CUDA
compilations.

Each CUDA compilation links in libdevice, a bitcode library provided by
nvidia as part of the CUDA distribution.  Without this patch, if we have
a user-function F that is compiled with -ffast-math that calls a
function G from libdevice, F will have the unsafe-fp-math=true (etc.)
attributes, but G will have no attributes.

Since F calls G, the inliner will merge G's attributes into F's.  It
considers the lack of an unsafe-fp-math=true attribute on G to be
tantamount to unsafe-fp-math=false, so it "merges" these by setting
unsafe-fp-math=false on F.

This then continues up the call graph, until every function that
(transitively) calls something in libdevice gets unsafe-fp-math=false
set, thus disabling fastmath in almost all CUDA code.

Reviewers: echristo

Subscribers: hfinkel, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D28538

llvm-svn: 293097

1 files changed, 25 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.h b/clang/lib/CodeGen/CodeGenModule.h
index 1715b3d..613e300 100644
--- a/clang/lib/CodeGen/CodeGenModule.h
+++ b/clang/lib/CodeGen/CodeGenModule.h
@@ -1022,6 +1022,25 @@ public:
                               CGCalleeInfo CalleeInfo, AttributeListType &PAL,
                               unsigned &CallingConv, bool AttrOnCallSite);
 
+  /// Adds attributes to F according to our CodeGenOptions and LangOptions, as
+  /// though we had emitted it ourselves.  We remove any attributes on F that
+  /// conflict with the attributes we add here.
+  ///
+  /// This is useful for adding attrs to bitcode modules that you want to link
+  /// with but don't control, such as CUDA's libdevice.  When linking with such
+  /// a bitcode library, you might want to set e.g. its functions'
+  /// "unsafe-fp-math" attribute to match the attr of the functions you're
+  /// codegen'ing.  Otherwise, LLVM will interpret the bitcode module's lack of
+  /// unsafe-fp-math attrs as tantamount to unsafe-fp-math=false, and then LLVM
+  /// will propagate unsafe-fp-math=false up to every transitive caller of a
+  /// function in the bitcode library!
+  ///
+  /// With the exception of fast-math attrs, this will only make the attributes
+  /// on the function more conservative.  But it's unsafe to call this on a
+  /// function which relies on particular fast-math attributes for correctness.
+  /// It's up to you to ensure that this is safe.
+  void AddDefaultFnAttrs(llvm::Function &F);
+
   // Fills in the supplied string map with the set of target features for the
   // passed in function.
   void getFunctionFeatureMap(llvm::StringMap<bool> &FeatureMap,
@@ -1303,6 +1322,12 @@ private:
   /// Check whether we can use a "simpler", more core exceptions personality
   /// function.
   void SimplifyPersonality();
+
+  /// Helper function for ConstructAttributeList and AddDefaultFnAttrs.
+  /// Constructs an AttrList for a function with the given properties.
+  void ConstructDefaultFnAttrList(StringRef Name, bool HasOptnone,
+                                  bool AttrOnCallSite,
+                                  llvm::AttrBuilder &FuncAttrs);
 };
 }  // end namespace CodeGen
 }  // end namespace clang