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, 6 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
index 81a429fe618..6f3d798f704 100644
--- a/llvm/lib/Transforms/Scalar/SCCP.cpp
+++ b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -1133,8 +1133,12 @@ void SCCPSolver::visitLoadInst(LoadInst &I) {
   Constant *Ptr = PtrVal.getConstant();
 
   // load null is undefined.
-  if (isa<ConstantPointerNull>(Ptr) && I.getPointerAddressSpace() == 0)
-    return;
+  if (isa<ConstantPointerNull>(Ptr)) {
+    if (NullPointerIsDefined(I.getFunction(), I.getPointerAddressSpace()))
+      return (void)markOverdefined(IV, &I);
+    else
+      return;
+  }
 
   // Transform load (constant global) into the value loaded.
   if (auto *GV = dyn_cast<GlobalVariable>(Ptr)) {