[PATCH] Prevent MDNode's RAUW from introducing a reference to a void Value.

[Duncan P. N. Exon Smith]

> On Oct 16, 2014, at 3:54 PM, Frederic Riss <friss at apple.com> wrote:
> 
> If you look at the added testcase, it cast a void (*func)() to a int (*func)(). The int typed result is referenced in a debug info metadata node (an argument to the dbg.value intrinsic). Then instcombine would remove the bitcast and turn the int returning function into a void returning function that gets RAUWd over the value in the MDNode. And at this point in the current code you have invalid IR. 
> 
> The code in the testcase is reduced from a huge LTO link that encountered this situation in a much more complicated setup.
> 
> http://reviews.llvm.org/D5828

Weird.  This LGTM (after fixing a few super minor whitespace changes);
not sure if Eric is still looking.

> Index: lib/IR/Metadata.cpp
> ===================================================================
> --- lib/IR/Metadata.cpp
> +++ lib/IR/Metadata.cpp
> @@ -90,7 +90,11 @@
>  }
>  
>  void MDNodeOperand::allUsesReplacedWith(Value *NV) {
> -  getParent()->replaceOperand(this, NV);
> +  // Void type operands aren't allowed anyway.

"anyway" is redundant.  I'd just say:

    // Void type operands aren't allowed.

> +  if (NV->getType()->isVoidTy())
> +    deleted();
> +  else
> +    getParent()->replaceOperand(this, NV);
>  }
>  
>  //===----------------------------------------------------------------------===//
> Index: test/DebugInfo/voidMDNode.ll
> ===================================================================
> --- /dev/null
> +++ test/DebugInfo/voidMDNode.ll
> @@ -0,0 +1,65 @@
> +; RUN: opt -sroa -instcombine < %s | FileCheck %s
> +
> +; IR generated from (the obvisouly wrong):

s/obvisouly/obviously/

> +; void foo(void) {}
> +; void baz() {
> +;	int deadvar = ((int (*)(void))&foo)();
> +; }
> +
> +; Function Attrs: nounwind ssp uwtable
> +define void @foo() #0 {
> +entry:
> +  ret void, !dbg !17
> +}
> +
> +; Function Attrs: nounwind ssp uwtable
> +define void @baz() #0 {
> +entry:
> +  %deadvar = alloca i32, align 4
> +; This dbg.declare will be turned in a dbg.value describing the bellow
> +; store by sroa. The created dbg.value first argument will be the %call
> +; value.
> +  call void @llvm.dbg.declare(metadata !{i32* %deadvar}, metadata !18, metadata !19), !dbg !20
> +  %call = call i32 bitcast (void ()* @foo to i32 ()*)(), !dbg !21
> +; When instcombine removes the bitcast in the call instruction, it will
> +; RAUW the %call value with the replacement instruction that is of void
> +; type. Check that the dbg.value get's nullified rather that having a
> +; bad reference to a void Value.
> +;CHECK-NOT: <badref>

Usually we have a space after the `;` for CHECKs.

    ; CHECK-NOT: <badref>

> +  store i32 %call, i32* %deadvar, align 4, !dbg !21
> +  ret void, !dbg !22
> +}
> +
> +; Function Attrs: nounwind readnone
> +declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
> +
> +attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
> +attributes #1 = { nounwind readnone }
> +
> +!llvm.dbg.cu = !{!0}
> +!llvm.module.flags = !{!14, !15}
> +!llvm.ident = !{!16}
> +
> +!0 = metadata !{metadata !"0x11\0012\00clang version 3.6.0 \000\00\000\00\001", metadata !1, metadata !2, metadata !3, metadata !8, metadata !2, metadata !2} ; [ DW_TAG_compile_unit ] [/tmp/voidmetadata.c] [DW_LANG_C99]
> +!1 = metadata !{metadata !"voidmetadata.c", metadata !"/tmp"}
> +!2 = metadata !{}
> +!3 = metadata !{metadata !4}
> +!4 = metadata !{metadata !"0xf\00\000\0064\0064\000\000", null, null, metadata !5} ; [ DW_TAG_pointer_type ] [line 0, size 64, align 64, offset 0] [from ]
> +!5 = metadata !{metadata !"0x15\00\000\000\000\000\000\000", null, null, null, metadata !6, null, null, null} ; [ DW_TAG_subroutine_type ] [line 0, size 0, align 0, offset 0] [from ]
> +!6 = metadata !{metadata !7}
> +!7 = 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]
> +!8 = metadata !{metadata !9, metadata !13}
> +!9 = metadata !{metadata !"0x2e\00foo\00foo\00\002\000\001\000\000\00256\000\002", metadata !1, metadata !10, metadata !11, null, void ()* @foo, null, null, metadata !2} ; [ DW_TAG_subprogram ] [line 2] [def] [foo]
> +!10 = metadata !{metadata !"0x29", metadata !1}   ; [ DW_TAG_file_type ] [/tmp/voidmetadata.c]
> +!11 = metadata !{metadata !"0x15\00\000\000\000\000\000\000", null, null, null, metadata !12, null, null, null} ; [ DW_TAG_subroutine_type ] [line 0, size 0, align 0, offset 0] [from ]
> +!12 = metadata !{null}
> +!13 = metadata !{metadata !"0x2e\00baz\00baz\00\004\000\001\000\000\000\000\004", metadata !1, metadata !10, metadata !11, null, void ()* @baz, null, null, metadata !2} ; [ DW_TAG_subprogram ] [line 4] [def] [baz]
> +!14 = metadata !{i32 2, metadata !"Dwarf Version", i32 2}
> +!15 = metadata !{i32 2, metadata !"Debug Info Version", i32 2}
> +!16 = metadata !{metadata !"clang version 3.6.0 "}
> +!17 = metadata !{i32 2, i32 17, metadata !9, null}
> +!18 = metadata !{metadata !"0x100\00deadvar\005\000", metadata !13, metadata !10, metadata !7} ; [ DW_TAG_auto_variable ] [deadvar] [line 5]
> +!19 = metadata !{metadata !"0x102"}               ; [ DW_TAG_expression ]
> +!20 = metadata !{i32 5, i32 6, metadata !13, null}
> +!21 = metadata !{i32 5, i32 16, metadata !13, null}
> +!22 = metadata !{i32 6, i32 1, metadata !13, null}
>