[Attributes] Remove nonnull from UB-implying attributes

From LangRef: > if the parameter or return pointer is null, poison value is > returned or passed instead. The nonnull attribute should be > combined with the noundef attribute to ensure a pointer is not > null or otherwise the behavior is undefined. Dropping noundef is sufficient to prevent UB. Including nonnull in this method just muddies the semantics.

Author: nikic

llvm/lib/IR/Attributes.cpp

@@ -1340,7 +1340,6 @@ AttributeList::removeParamUndefImplyingAttributes(LLVMContext &C,
  unsigned ArgNo) const {
  AttrBuilder B;
  B.addAttribute(Attribute::NoUndef);
- B.addAttribute(Attribute::NonNull);
  B.addDereferenceableAttr(1);
  B.addDereferenceableOrNullAttr(1);
  return removeParamAttributes(C, ArgNo, B);

llvm/test/Transforms/DeadArgElim/NoundefAttrs.ll

@@ -1,18 +1,18 @@
 ; RUN: opt -deadargelim -S < %s | FileCheck %s
 
-; If caller is changed to pass in undef, noundef and related attributes
-; should be deleted.
+; If caller is changed to pass in undef, noundef, dereferenceable and other
+; attributes that imply immediate undefined behavior should be delete.
+; Other attributes like nonnull, which only imply poison, can be safely kept.
 
-
-; CHECK: define i64 @bar(i64* %0, i64 %1)
+; CHECK: define i64 @bar(i64* nonnull %0, i64 %1)
 define i64 @bar(i64* nonnull dereferenceable(8) %0, i64 %1) {
 entry:
  %2 = add i64 %1, 8
  ret i64 %2
 }
 
 define i64 @foo(i64* %p, i64 %v) {
-; CHECK: %retval = call i64 @bar(i64* undef, i64 %v)
+; CHECK: %retval = call i64 @bar(i64* nonnull undef, i64 %v)
  %retval = call i64 @bar(i64* nonnull dereferenceable(8) %p, i64 %v)
  ret i64 %retval
 }

llvm/test/Transforms/InstCombine/unused-nonnull.ll

@@ -9,7 +9,7 @@ target triple = "x86_64-unknown-linux-gnu"
 
 define i32 @main(i32 %argc, i8** %argv) #0 {
 ; CHECK-LABEL: define {{[^@]+}}@main
-; CHECK-SAME: (i32 [[ARGC:%.*]], i8** nocapture readnone [[ARGV:%.*]]) local_unnamed_addr #0
+; CHECK-SAME: (i32 [[ARGC:%.*]], i8** nocapture readnone [[ARGV:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
 ; CHECK-NEXT: entry:
 ; CHECK-NEXT: [[TMP0:%.*]] = icmp slt i32 [[ARGC]], 2
 ; CHECK-NEXT: [[SPEC_SELECT:%.*]] = select i1 [[TMP0]], i32 0, i32 [[ARGC]]
@@ -37,7 +37,7 @@ done:
 
 define i32 @compute(i8* noundef nonnull %ptr, i32 %x) #1 {
 ; CHECK-LABEL: define {{[^@]+}}@compute
-; CHECK-SAME: (i8* nocapture readnone [[PTR:%.*]], i32 returned [[X:%.*]]) local_unnamed_addr #1
+; CHECK-SAME: (i8* nocapture nonnull readnone [[PTR:%.*]], i32 returned [[X:%.*]]) local_unnamed_addr #[[ATTR1:[0-9]+]] {
 ; CHECK-NEXT: ret i32 [[X]]
 ;
  ret i32 %x