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, 5 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index c15f649e9c4..061ce8f0bd1 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -2120,9 +2120,12 @@ computePointerICmp(const DataLayout &DL, const TargetLibraryInfo *TLI,
       ConstantInt *LHSOffsetCI = dyn_cast<ConstantInt>(LHSOffset);
       ConstantInt *RHSOffsetCI = dyn_cast<ConstantInt>(RHSOffset);
       uint64_t LHSSize, RHSSize;
+      ObjectSizeOpts Opts;
+      Opts.NullIsUnknownSize =
+          NullPointerIsDefined(cast<AllocaInst>(LHS)->getFunction());
       if (LHSOffsetCI && RHSOffsetCI &&
-          getObjectSize(LHS, LHSSize, DL, TLI) &&
-          getObjectSize(RHS, RHSSize, DL, TLI)) {
+          getObjectSize(LHS, LHSSize, DL, TLI, Opts) &&
+          getObjectSize(RHS, RHSSize, DL, TLI, Opts)) {
         const APInt &LHSOffsetValue = LHSOffsetCI->getValue();
         const APInt &RHSOffsetValue = RHSOffsetCI->getValue();
         if (!LHSOffsetValue.isNegative() &&