[clang-cl] Don't emit dllexport inline functions etc. from pch files (PR37801)
With MSVC, PCH files are created along with an object file that needs to
be linked into the final library or executable. That object file
contains the code generated when building the headers. In particular, it
will include definitions of inline dllexport functions, and because they
are emitted in this object file, other files using the PCH do not need
to emit them. See the bug for an example.
This patch makes clang-cl match MSVC's behaviour in this regard, causing
significant compile-time savings when building dlls using precompiled
headers.
For example, in a 64-bit optimized shared library build of Chromium with
PCH, it reduces the binary size and compile time of
stroke_opacity_custom.obj from 9315564 bytes to 3659629 bytes and 14.6
to 6.63 s. The wall-clock time of building blink_core.dll goes from
38m41s to 22m33s. ("user" time goes from 1979m to 1142m).
Differential Revision: https://reviews.llvm.org/D48426
llvm-svn: 335466
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 96b7b56..43577e8 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -9553,6 +9553,29 @@
else
return false;
+ if (D->isFromASTFile() && !LangOpts.BuildingPCHWithObjectFile) {
+ assert(getExternalSource() && "It's from an AST file; must have a source.");
+ // On Windows, PCH files are built together with an object file. If this
+ // declaration comes from such a PCH and DeclMustBeEmitted would return
+ // true, it would have returned true and the decl would have been emitted
+ // into that object file, so it doesn't need to be emitted here.
+ // Note that decls are still emitted if they're referenced, as usual;
+ // DeclMustBeEmitted is used to decide whether a decl must be emitted even
+ // if it's not referenced.
+ //
+ // Explicit template instantiation definitions are tricky. If there was an
+ // explicit template instantiation decl in the PCH before, it will look like
+ // the definition comes from there, even if that was just the declaration.
+ // (Explicit instantiation defs of variable templates always get emitted.)
+ bool IsExpInstDef =
+ isa<FunctionDecl>(D) &&
+ cast<FunctionDecl>(D)->getTemplateSpecializationKind() ==
+ TSK_ExplicitInstantiationDefinition;
+
+ if (getExternalSource()->DeclIsFromPCHWithObjectFile(D) && !IsExpInstDef)
+ return false;
+ }
+
// If this is a member of a class template, we do not need to emit it.
if (D->getDeclContext()->isDependentContext())
return false;
@@ -9573,7 +9596,7 @@
// Constructors and destructors are required.
if (FD->hasAttr<ConstructorAttr>() || FD->hasAttr<DestructorAttr>())
return true;
-
+
// The key function for a class is required. This rule only comes
// into play when inline functions can be key functions, though.
if (getTargetInfo().getCXXABI().canKeyFunctionBeInline()) {
@@ -9594,7 +9617,7 @@
// Implicit template instantiations can also be deferred in C++.
return !isDiscardableGVALinkage(Linkage);
}
-
+
const auto *VD = cast<VarDecl>(D);
assert(VD->isFileVarDecl() && "Expected file scoped var");