[clang]: 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 as the function attribute
"null-pointer-is-valid"="true".
This CL only adds the attribute on the function.
It also strips "nonnull" attributes from function arguments but
keeps the related warnings unchanged.

Corresponding LLVM change rL336613 already updated the
optimizations to not treat null pointer dereferencing
as undefined if the attribute is present.

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

Reviewed By: jyknight

Subscribers: drinkcat, xbolva00, cfe-commits

Differential Revision: https://reviews.llvm.org/D47894

llvm-svn: 337433

3 files changed, 20 insertions, 6 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 07fb6d56dd7..fcc8a3e5f64 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -1734,6 +1734,8 @@ void CodeGenModule::ConstructDefaultFnAttrList(StringRef Name, bool HasOptnone,
     FuncAttrs.addAttribute("less-precise-fpmad",
                            llvm::toStringRef(CodeGenOpts.LessPreciseFPMAD));
 
+    if (CodeGenOpts.NullPointerIsValid)
+      FuncAttrs.addAttribute("null-pointer-is-valid", "true");
     if (!CodeGenOpts.FPDenormalMode.empty())
       FuncAttrs.addAttribute("denormal-fp-math", CodeGenOpts.FPDenormalMode);
 
@@ -1867,7 +1869,8 @@ void CodeGenModule::ConstructAttributeList(
     }
     if (TargetDecl->hasAttr<RestrictAttr>())
       RetAttrs.addAttribute(llvm::Attribute::NoAlias);
-    if (TargetDecl->hasAttr<ReturnsNonNullAttr>())
+    if (TargetDecl->hasAttr<ReturnsNonNullAttr>() &&
+        !CodeGenOpts.NullPointerIsValid)
       RetAttrs.addAttribute(llvm::Attribute::NonNull);
     if (TargetDecl->hasAttr<AnyX86NoCallerSavedRegistersAttr>())
       FuncAttrs.addAttribute("no_caller_saved_registers");
@@ -1974,7 +1977,8 @@ void CodeGenModule::ConstructAttributeList(
     if (!PTy->isIncompleteType() && PTy->isConstantSizeType())
       RetAttrs.addDereferenceableAttr(getContext().getTypeSizeInChars(PTy)
                                         .getQuantity());
-    else if (getContext().getTargetAddressSpace(PTy) == 0)
+    else if (getContext().getTargetAddressSpace(PTy) == 0 &&
+             !CodeGenOpts.NullPointerIsValid)
       RetAttrs.addAttribute(llvm::Attribute::NonNull);
   }
 
@@ -2083,7 +2087,8 @@ void CodeGenModule::ConstructAttributeList(
       if (!PTy->isIncompleteType() && PTy->isConstantSizeType())
         Attrs.addDereferenceableAttr(getContext().getTypeSizeInChars(PTy)
                                        .getQuantity());
-      else if (getContext().getTargetAddressSpace(PTy) == 0)
+      else if (getContext().getTargetAddressSpace(PTy) == 0 &&
+               !CodeGenOpts.NullPointerIsValid)
         Attrs.addAttribute(llvm::Attribute::NonNull);
     }
 
@@ -2343,7 +2348,8 @@ void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
 
         if (const ParmVarDecl *PVD = dyn_cast<ParmVarDecl>(Arg)) {
           if (getNonNullAttr(CurCodeDecl, PVD, PVD->getType(),
-                             PVD->getFunctionScopeIndex()))
+                             PVD->getFunctionScopeIndex()) &&
+              !CGM.getCodeGenOpts().NullPointerIsValid)
             AI->addAttr(llvm::Attribute::NonNull);
 
           QualType OTy = PVD->getOriginalType();
@@ -2362,7 +2368,8 @@ void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
                 Attrs.addDereferenceableAttr(
                   getContext().getTypeSizeInChars(ETy).getQuantity()*ArrSize);
                 AI->addAttrs(Attrs);
-              } else if (getContext().getTargetAddressSpace(ETy) == 0) {
+              } else if (getContext().getTargetAddressSpace(ETy) == 0 &&
+                         !CGM.getCodeGenOpts().NullPointerIsValid) {
                 AI->addAttr(llvm::Attribute::NonNull);
               }
             }
@@ -2372,7 +2379,8 @@ void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
             // we can't use the dereferenceable attribute, but in addrspace(0)
             // we know that it must be nonnull.
             if (ArrTy->getSizeModifier() == VariableArrayType::Static &&
-                !getContext().getTargetAddressSpace(ArrTy->getElementType()))
+                !getContext().getTargetAddressSpace(ArrTy->getElementType()) &&
+                !CGM.getCodeGenOpts().NullPointerIsValid)
               AI->addAttr(llvm::Attribute::NonNull);
           }
 
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 4db4dc19a8b..e23ff16699f 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3348,6 +3348,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                    options::OPT_fno_merge_all_constants, false))
     CmdArgs.push_back("-fmerge-all-constants");
 
+  if (Args.hasFlag(options::OPT_fno_delete_null_pointer_checks,
+                   options::OPT_fdelete_null_pointer_checks, false))
+    CmdArgs.push_back("-fno-delete-null-pointer-checks");
+
   // LLVM Code Generator Options.
 
   if (Args.hasArg(options::OPT_frewrite_map_file) ||
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index c26d5fb79b3..9bb673d1e77 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -746,6 +746,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
 
   Opts.NoUseJumpTables = Args.hasArg(OPT_fno_jump_tables);
 
+  Opts.NullPointerIsValid = Args.hasArg(OPT_fno_delete_null_pointer_checks);
+
   Opts.ProfileSampleAccurate = Args.hasArg(OPT_fprofile_sample_accurate);
 
   Opts.PrepareForLTO = Args.hasArg(OPT_flto, OPT_flto_EQ);