llvm: Add support for "-fno-delete-null-pointer-checks"

Summary:
Support for this option is needed for building Linux kernel.
This is a very frequently requested feature by kernel developers.

More details : https://lkml.org/lkml/2018/4/4/601

GCC option description for -fdelete-null-pointer-checks:
This Assume that programs cannot safely dereference null pointers,
and that no code or data element resides at address zero.

-fno-delete-null-pointer-checks is the inverse of this implying that
null pointer dereferencing is not undefined.

This feature is implemented in LLVM IR in this CL as the function attribute
"null-pointer-is-valid"="true" in IR (Under review at D47894).
The CL updates several passes that assumed null pointer dereferencing is
undefined to not optimize when the "null-pointer-is-valid"="true"
attribute is present.

Reviewers: t.p.northover, efriedma, jyknight, chandlerc, rnk, srhines, void, george.burgess.iv

Reviewed By: efriedma, george.burgess.iv

Subscribers: eraman, haicheng, george.burgess.iv, drinkcat, theraven, reames, sanjoy, xbolva00, llvm-commits

Differential Revision: https://reviews.llvm.org/D47895

llvm-svn: 336613

1 files changed, 18 insertions, 1 deletions

diff --git a/llvm/lib/IR/Function.cpp b/llvm/lib/IR/Function.cpp
index 49582c4..aba329b 100644
--- a/llvm/lib/IR/Function.cpp
+++ b/llvm/lib/IR/Function.cpp
@@ -79,7 +79,8 @@ bool Argument::hasNonNullAttr() const {
   if (getParent()->hasParamAttribute(getArgNo(), Attribute::NonNull))
     return true;
   else if (getDereferenceableBytes() > 0 &&
-           getType()->getPointerAddressSpace() == 0)
+           !NullPointerIsDefined(getParent(),
+                                 getType()->getPointerAddressSpace()))
     return true;
   return false;
 }
@@ -1415,3 +1416,19 @@ Optional<StringRef> Function::getSectionPrefix() const {
   }
   return None;
 }
+
+bool Function::nullPointerIsDefined() const {
+  return getFnAttribute("null-pointer-is-valid")
+          .getValueAsString()
+          .equals("true");
+}
+
+bool llvm::NullPointerIsDefined(const Function *F, unsigned AS) {
+  if (F && F->nullPointerIsDefined())
+    return true;
+
+  if (AS != 0)
+    return true;
+
+  return false;
+}