[ArgPromotion] Add DW_CC_nocall to DISubprogram

ArgumentPromotion pass may change function signatures. If this happens
and debuginfo is enabled, let us add DW_CC_nocall to debuginfo so it is
clear that the function signature has changed.
DeadArgumentElimination ([1]) has similar implementation.

Also fix an ArgumentPromotion test due to adding DW_CC_nocall to
debuginfo.

  [1] 340b0ca

Author: Yonghong Song

llvm/lib/Transforms/IPO/ArgumentPromotion.cpp

@@ -50,6 +50,7 @@
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Dominators.h"
@@ -432,6 +433,16 @@ doPromotion(Function *F, FunctionAnalysisManager &FAM,
     PromoteMemToReg(Allocas, DT, &AC);
   }
 
+  // If argument(s) are dead (hence removed) or promoted, probably the function
+  // does not follow standard calling convention anymore. Add DW_CC_nocall to
+  // DISubroutineType to inform debugger that it may not be safe to call this
+  // function.
+  DISubprogram *SP = NF->getSubprogram();
+  if (SP) {
+    auto Temp = SP->getType()->cloneWithCC(llvm::dwarf::DW_CC_nocall);
+    SP->replaceType(MDNode::replaceWithPermanent(std::move(Temp)));
+  }
+
   return NF;
 }
 

llvm/test/Transforms/ArgumentPromotion/dbg.ll

@@ -53,7 +53,11 @@ define void @caller(ptr %Y, ptr %P) {
 
 !0 = !{i32 2, !"Debug Info Version", i32 3}
 !1 = !DILocation(line: 8, scope: !2)
-!2 = distinct !DISubprogram(name: "test", file: !5, line: 3, isLocal: true, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: false, unit: !3, scopeLine: 3, scope: null)
+!2 = distinct !DISubprogram(name: "test", file: !5, line: 3, type: !7, isLocal: true, isDefinition: true, flags: DIFlagPrototyped, isOptimized: false, unit: !3, scopeLine: 3, scope: null)
 !3 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.5.0 ", isOptimized: false, emissionKind: LineTablesOnly, file: !5)
 !5 = !DIFile(filename: "test.c", directory: "")
 !6 = !DILocation(line: 9, scope: !2)
+!7 = !DISubroutineType(types: !8)
+!8 = !{null, !9}
+!9 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !10)
+!10 = !DIBasicType(name: "int",  size: 32, encoding: DW_ATE_signed)