Hi rsmith, chandlerc, nlewycky,
This is the Clang companion patch to http://reviews.llvm.org/D4449 which adds a
dereferencable function parameter attribute (like the nonnull attribute, but
specifying that the pointer is dereferencable like the value of an alloca is
dereferencable). This patch causes the dereferencable attribute to appear on
reference parameters (and return values) just like nonnull does currently.
As mentioned in the llvm patch summary, my motivating use case is C++
references that are passes as function parameters and used in loops. For
example,
void foo(int * __restrict__ a, int * __restrict__ b, int &c, int n) {
for (int i = 0; i < n; ++i)
if (a[i] > 0)
a[i] = c*b[i];
}
Currently, we generate a load of 'c' in every loop iteration because we can't
prove that there is no control dependence on the dereferencability of 'c' from
the if condition. But because 'c' is a reference, we know it must point to
something, and since nothing else in the loop can alias it, it is safe to load
it in the preheader.
http://reviews.llvm.org/D4450
Files:
lib/CodeGen/CGCall.cpp
Index: lib/CodeGen/CGCall.cpp
===================================================================
--- lib/CodeGen/CGCall.cpp
+++ lib/CodeGen/CGCall.cpp
@@ -1200,8 +1200,10 @@
llvm_unreachable("Invalid ABI kind for return argument");
}
- if (RetTy->isReferenceType())
+ if (RetTy->isReferenceType()) {
+ RetAttrs.addAttribute(llvm::Attribute::Dereferencable);
RetAttrs.addAttribute(llvm::Attribute::NonNull);
+ }
if (RetAttrs.hasAttributes())
PAL.push_back(llvm::
@@ -1291,8 +1293,10 @@
}
}
- if (ParamType->isReferenceType())
+ if (ParamType->isReferenceType()) {
+ Attrs.addAttribute(llvm::Attribute::Dereferencable);
Attrs.addAttribute(llvm::Attribute::NonNull);
+ }
if (Attrs.hasAttributes())
PAL.push_back(llvm::AttributeSet::get(getLLVMContext(), Index, Attrs));
Index: lib/CodeGen/CGCall.cpp
===================================================================
--- lib/CodeGen/CGCall.cpp
+++ lib/CodeGen/CGCall.cpp
@@ -1200,8 +1200,10 @@
llvm_unreachable("Invalid ABI kind for return argument");
}
- if (RetTy->isReferenceType())
+ if (RetTy->isReferenceType()) {
+ RetAttrs.addAttribute(llvm::Attribute::Dereferencable);
RetAttrs.addAttribute(llvm::Attribute::NonNull);
+ }
if (RetAttrs.hasAttributes())
PAL.push_back(llvm::
@@ -1291,8 +1293,10 @@
}
}
- if (ParamType->isReferenceType())
+ if (ParamType->isReferenceType()) {
+ Attrs.addAttribute(llvm::Attribute::Dereferencable);
Attrs.addAttribute(llvm::Attribute::NonNull);
+ }
if (Attrs.hasAttributes())
PAL.push_back(llvm::AttributeSet::get(getLLVMContext(), Index, Attrs));
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