Safely generate new loop metadata node

[dpeixott]

Summary

Polly was accidently modifying a debug info metadata node when
attempting to generate a new unique metadata node for the loop id.
The problem was that we had dwarf metadata that referred to a
metadata node with a null value, like this:

!6 = ... some dwarf metadata referring to !7 ...
!7 = {null}

When we attempt to generate a new metadata node, we reserve the
first space for self-referential node by setting the first argument
to null and then mutating the node later to refer to itself.
However, because the nodes are uniqued based on pointer values, when
we get the new metadata node it actually referred to an existing
node (!7 in the example). When we went to modify the metadata to
point to itself, we were accidently mutating the dwarf metatdata. We
ended up in this situation:

!6 = ... some dwarf metadata referring to !7 ...
!7 = {!7}

and this causes an assert when generating the debug info. The fix is
simple, we just need to use a unique value when getting a new
metadata node. The MDNode::getTemporary() provides exactly the API
we need (and it is used in clang to generate the unique nodes).

[jdoerfert]

LGTM

[dpeixott]

Closed by commit rL221550 (authored by @dpeixott).

polly/trunk/lib/CodeGen/IRBuilder.cpp

 ///
 ///    '!n = metadata !{metadata !n, arg0, arg1}'
 ///
 /// @return The self referencing id metadata node.
 static MDNode *getID(LLVMContext &Ctx, Value *arg0 = nullptr,
                      Value *arg1 = nullptr) {
   MDNode *ID;
   SmallVector<Value *, 3> Args;
-  Args.push_back(nullptr);
+  // Use a temporary node to safely create a unique pointer for the first arg.
+  MDNode *TempNode = MDNode::getTemporary(Ctx, None);
+  // Reserve operand 0 for loop id self reference.
+  Args.push_back(TempNode);
 
   if (arg0)
     Args.push_back(arg0);
   if (arg1)
     Args.push_back(arg1);
 
   ID = MDNode::get(Ctx, Args);
   ID->replaceOperandWith(0, ID);
+  MDNode::deleteTemporary(TempNode);
   return ID;
 }
 
 ScopAnnotator::ScopAnnotator() : SE(nullptr), AliasScopeDomain(nullptr) {}
 
 void ScopAnnotator::buildAliasScopes(Scop &S) {
   SE = S.getSE();
 

polly/trunk/test/Isl/CodeGen/LoopParallelMD/do_not_mutate_debug_info.ll

+; This test checks that we do not accidently mutate the debug info when
+; inserting loop parallel metadata.
+; RUN: opt %loadPolly < %s  -S -polly -polly-codegen-isl -polly-ast-detect-parallel | FileCheck %s
+; CHECK-NOT: !7 = metadata !{metadata !7}
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@A = common global i32* null, align 8
+
+; Function Attrs: nounwind uwtable
+define void @foo() {
+entry:
+  tail call void @llvm.dbg.value(metadata !18, i64 0, metadata !9, metadata !19), !dbg !20
+  %0 = load i32** @A, align 8, !dbg !21, !tbaa !23
+  br label %for.body, !dbg !27
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i32* %0, i64 %indvars.iv, !dbg !21
+  %1 = load i32* %arrayidx, align 4, !dbg !21, !tbaa !30
+  %add = add nsw i32 %1, 1, !dbg !21
+  store i32 %add, i32* %arrayidx, align 4, !dbg !21, !tbaa !30
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1, !dbg !27
+  %exitcond = icmp eq i64 %indvars.iv, 1, !dbg !27
+  br i1 %exitcond, label %for.end, label %for.body, !dbg !27
+
+for.end:                                          ; preds = %for.body
+  ret void, !dbg !32
+}
+
+; Function Attrs: nounwind readnone
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata)
+
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!15, !16}
+!llvm.ident = !{!17}
+
+!0 = metadata !{metadata !"0x11\0012\00clang version 3.6.0 \001\00\000\00\001", metadata !1, metadata !2, metadata !2, metadata !3, metadata !12, metadata !2} ; [ DW_TAG_compile_unit ] [/local/mnt/workspace/build/tip-Release/t2.c] [DW_LANG_C99]
+!1 = metadata !{metadata !"t2.c", metadata !"/local/mnt/workspace/build/tip-Release"}
+!2 = metadata !{}
+!3 = metadata !{metadata !4}
+!4 = metadata !{metadata !"0x2e\00foo\00foo\00\003\000\001\000\000\000\001\003", metadata !1, metadata !5, metadata !6, null, void ()* @foo, null, null, metadata !8} ; [ DW_TAG_subprogram ] [line 3] [def] [foo]
+!5 = metadata !{metadata !"0x29", metadata !1}    ; [ DW_TAG_file_type ] [/local/mnt/workspace/build/tip-Release/t2.c]
+!6 = metadata !{metadata !"0x15\00\000\000\000\000\000\000", null, null, null, metadata !7, null, null, null} ; [ DW_TAG_subroutine_type ] [line 0, size 0, align 0, offset 0] [from ]
+!7 = metadata !{null}
+!8 = metadata !{metadata !9}
+!9 = metadata !{metadata !"0x100\00i\004\000", metadata !10, metadata !5, metadata !11} ; [ DW_TAG_auto_variable ] [i] [line 4]
+!10 = metadata !{metadata !"0xb\004\003\000", metadata !1, metadata !4} ; [ DW_TAG_lexical_block ] [/local/mnt/workspace/build/tip-Release/t2.c]
+!11 = metadata !{metadata !"0x24\00int\000\0032\0032\000\000\005", null, null} ; [ DW_TAG_base_type ] [int] [line 0, size 32, align 32, offset 0, enc DW_ATE_signed]
+!12 = metadata !{metadata !13}
+!13 = metadata !{metadata !"0x34\00A\00A\00\002\000\001", null, metadata !5, metadata !14, i32** @A, null} ; [ DW_TAG_variable ] [A] [line 2] [def]
+!14 = metadata !{metadata !"0xf\00\000\0064\0064\000\000", null, null, metadata !11} ; [ DW_TAG_pointer_type ] [line 0, size 64, align 64, offset 0] [from int]
+!15 = metadata !{i32 2, metadata !"Dwarf Version", i32 4}
+!16 = metadata !{i32 2, metadata !"Debug Info Version", i32 2}
+!17 = metadata !{metadata !"clang version 3.6.0 "}
+!18 = metadata !{i32 0}
+!19 = metadata !{metadata !"0x102"}               ; [ DW_TAG_expression ]
+!20 = metadata !{i32 4, i32 12, metadata !10, null}
+!21 = metadata !{i32 5, i32 5, metadata !22, null}
+!22 = metadata !{metadata !"0xb\004\003\001", metadata !1, metadata !10} ; [ DW_TAG_lexical_block ] [/local/mnt/workspace/build/tip-Release/t2.c]
+!23 = metadata !{metadata !24, metadata !24, i64 0}
+!24 = metadata !{metadata !"any pointer", metadata !25, i64 0}
+!25 = metadata !{metadata !"omnipotent char", metadata !26, i64 0}
+!26 = metadata !{metadata !"Simple C/C++ TBAA"}
+!27 = metadata !{i32 4, i32 3, metadata !28, null}
+!28 = metadata !{metadata !"0xb\002", metadata !1, metadata !29} ; [ DW_TAG_lexical_block ] [/local/mnt/workspace/build/tip-Release/t2.c]
+!29 = metadata !{metadata !"0xb\001", metadata !1, metadata !22} ; [ DW_TAG_lexical_block ] [/local/mnt/workspace/build/tip-Release/t2.c]
+!30 = metadata !{metadata !31, metadata !31, i64 0}
+!31 = metadata !{metadata !"int", metadata !25, i64 0}
+!32 = metadata !{i32 6, i32 1, metadata !4, null}