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, 8 insertions, 0 deletions

diff --git a/llvm/test/Transforms/GlobalOpt/heap-sra-phi.ll b/llvm/test/Transforms/GlobalOpt/heap-sra-phi.ll
index 995f9dc142c..770220dd07b 100644
--- a/llvm/test/Transforms/GlobalOpt/heap-sra-phi.ll
+++ b/llvm/test/Transforms/GlobalOpt/heap-sra-phi.ll
@@ -42,3 +42,11 @@ bb1:		; preds = %bb1, %bb1.thread
 bb2:		; preds = %bb1
 	ret i32 %tmp3
 }
+
+define void @bam(i64 %Size) nounwind noinline #0 {
+entry:
+        %0 = load %struct.foo*, %struct.foo** @X, align 4
+        ret void
+}
+
+attributes #0 = { "null-pointer-is-valid"="true" }