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, 9 insertions, 3 deletions

diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index 9be9d3adf69..90a8366d169 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -1500,8 +1500,12 @@ static ICmpInst::Predicate evaluateICmpRelation(Constant *V1, Constant *V2,
       assert(isa<ConstantPointerNull>(V2) && "Canonicalization guarantee!");
       // GlobalVals can never be null unless they have external weak linkage.
       // We don't try to evaluate aliases here.
+      // NOTE: We should not be doing this constant folding if null pointer
+      // is considered valid for the function. But currently there is no way to
+      // query it from the Constant type.
       if (!GV->hasExternalWeakLinkage() && !isa<GlobalAlias>(GV) &&
-          GV->getType()->getAddressSpace() == 0)
+          !NullPointerIsDefined(nullptr /* F */,
+                                GV->getType()->getAddressSpace()))
         return ICmpInst::ICMP_NE;
     }
   } else if (const BlockAddress *BA = dyn_cast<BlockAddress>(V1)) {
@@ -1731,7 +1735,8 @@ Constant *llvm::ConstantFoldCompareInstruction(unsigned short pred,
     if (const GlobalValue *GV = dyn_cast<GlobalValue>(C2))
       // Don't try to evaluate aliases.  External weak GV can be null.
       if (!isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage() &&
-          GV->getType()->getAddressSpace() == 0) {
+          !NullPointerIsDefined(nullptr /* F */,
+                                GV->getType()->getAddressSpace())) {
         if (pred == ICmpInst::ICMP_EQ)
           return ConstantInt::getFalse(C1->getContext());
         else if (pred == ICmpInst::ICMP_NE)
@@ -1742,7 +1747,8 @@ Constant *llvm::ConstantFoldCompareInstruction(unsigned short pred,
     if (const GlobalValue *GV = dyn_cast<GlobalValue>(C1))
       // Don't try to evaluate aliases.  External weak GV can be null.
       if (!isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage() &&
-          GV->getType()->getAddressSpace() == 0) {
+          !NullPointerIsDefined(nullptr /* F */,
+                                GV->getType()->getAddressSpace())) {
         if (pred == ICmpInst::ICMP_EQ)
           return ConstantInt::getFalse(C1->getContext());
         else if (pred == ICmpInst::ICMP_NE)