SimplifyAllocas: search through more bitcasts to find alloca for llvm.dbg.declare.

The webp/snes9x build was generating more intermediate
bitcasts than expected, so the fixup for making
llvm.dbg.declare's first argument be an alloca was
sometimes failing to find the alloca. I.e., some earlier
pass like instcombine was already violating the contract
that llvm.dbg.declare's first argument be an alloca or a
function argument.

Try to be more resilient and loop through more casts if
needed.

Technically llvm.dbg.declare's first argument doesn't
*have* to be an alloca:

http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-June/086647.html

However, ReplacePtrsWithInts does not handle converting
llvm.dbg.declare parameters which are bitcasts, and ends
up erasing the bitcasts. This causes us to drop debug
info. An alternative is to fix up ReplacePtrsWithInts
(TODO), but we need to make that work with DCE.
That is, the conversions inserted by ReplacePtrsWithInts
must not be used solely by llvm.dbg.declare, otherwise
DCE will delete those conversions and leave us with
a null argument to llvm.dbg.declare.

Example program state prior to SimplifyAllocas:

; Function Attrs: nounwind readonly
define internal fastcc i32 @png_sig_cmp(i8* readonly %sig, i32 %start, i32 %num_to_check) #3 {
entry:
  %png_signature = alloca i64, align 8, !dbg !68085
  %tmpcast = bitcast i64* %png_signature to [8 x i8]*, !dbg !68085
  call void @llvm.dbg.declare(metadata [8 x i8]* %tmpcast, metadata !68092, metadata !45244), !dbg !68085
  store i64 727905341920923785, i64* %png_signature, align 8, !dbg !68085
  %cmp = icmp ugt i32 %num_to_check, 8, !dbg !68093
  br i1 %cmp, label %if.end3, label %if.else, !dbg !68095

// ...
if.end6:
// ...
  %gep_int6 = ptrtoint [8 x i8]* %tmpcast to i32, !dbg !68108
// ...

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4203
[email protected]

Review URL: https://codereview.chromium.org/1166253003.

Author: Jan Voung

lib/Transforms/NaCl/SimplifyAllocas.cpp

@@ -18,7 +18,9 @@
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
 #include "llvm/Transforms/NaCl.h"
+#include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
+
 using namespace llvm;
 namespace {
 class SimplifyAllocas : public BasicBlockPass {
@@ -50,6 +52,21 @@ class SimplifyAllocas : public BasicBlockPass {
     return false;
   }
 
+  AllocaInst *findAllocaFromBC(BitCastInst *BCInst) {
+    Value *Op0 = BCInst->getOperand(0);
+    while (!llvm::isa<AllocaInst>(Op0)) {
+      if (auto *NextBC = llvm::dyn_cast<BitCastInst>(Op0)) {
+        Op0 = NextBC->getOperand(0);
+      } else {
+        dbgs() << "findAllocaFromBC encountered a non-bitcast intermediate val "
+               << *Op0 << " starting w/ BCInst " << *BCInst << "\n";
+        report_fatal_error(
+            "findAllocaFromBC encountered a non-bitcast intermediate");
+      }
+    }
+    return llvm::cast<AllocaInst>(Op0);
+  }
+
   bool runOnBasicBlock(BasicBlock &BB) override {
     bool Changed = false;
     for (BasicBlock::iterator I = BB.getFirstInsertionPt(), E = BB.end();
@@ -106,12 +123,11 @@ class SimplifyAllocas : public BasicBlockPass {
           // Sometimes dbg.declare points to an argument instead of an alloca.
           if (auto *VM = dyn_cast<ValueAsMetadata>(MV->getMetadata())) {
             if (auto *BCInst = dyn_cast<BitCastInst>(VM->getValue())) {
-              Value *CastSrc = BCInst->getOperand(0);
-              assert(isa<AllocaInst>(CastSrc));
+              AllocaInst *Alloca = findAllocaFromBC(BCInst);
               Call->setArgOperand(
                   0,
                   MetadataAsValue::get(Inst->getContext(),
-                                       ValueAsMetadata::get(CastSrc)));
+                                       ValueAsMetadata::get(Alloca)));
               Changed = true;
             }
           }

test/Transforms/NaCl/simplify-allocas.ll

@@ -138,31 +138,46 @@ define void @dyn_inst_alloca_array_zext(i8 %a) {
 declare void @llvm.dbg.declare(metadata, metadata, metadata)
 define void @debug_declare() {
   %var = alloca i32
-  call void @llvm.dbg.declare(metadata i32* %var, metadata !11, metadata !12), !dbg !13
+  call void @llvm.dbg.declare(metadata i32* %var, metadata !12, metadata !13), !dbg !14
   unreachable
 }
 ; Ensure that the first arg to dbg.declare points to the alloca, not the bitcast
 ; CHECK-LABEL: define void @debug_declare
 ; CHECK-NEXT: %var = alloca i8, i32 4
-; CHECK: call void @llvm.dbg.declare(metadata i8* %var, metadata !11, metadata !12), !dbg !13
+; CHECK: call void @llvm.dbg.declare(metadata i8* %var, metadata !12, metadata !13), !dbg !14
+
+define void @debug_declare_morecasts() {
+  %var = alloca i32, i32 2, align 8
+  %other_bc = bitcast i32* %var to i64*
+  %other_bc2 = bitcast i64* %other_bc to i16*
+  call void @llvm.dbg.declare(metadata i16* %other_bc2, metadata !15, metadata !13), !dbg !16
+  unreachable
+}
+; Ensure that the first arg to dbg.declare points to the alloca, not bitcasts
+; CHECK-LABEL: define void @debug_declare_morecasts
+; CHECK-NEXT: %var = alloca i8, i32 8, align 8
+; CHECK: call void @llvm.dbg.declare(metadata i8* %var, metadata !15, metadata !13), !dbg !16
 
 !llvm.dbg.cu = !{!0}
-!llvm.module.flags = !{!8, !9}
-!llvm.ident = !{!10}
+!llvm.module.flags = !{!9, !10}
+!llvm.ident = !{!11}
 
 ; CHECK: !4 = !MDSubprogram(name: "debug_declare", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: false, function: void ()* @debug_declare, variables: !2)
 
 !0 = !MDCompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.7.0 (trunk 235150) (llvm/trunk 235152)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, retainedTypes: !2, subprograms: !3, globals: !2, imports: !2)
 !1 = !MDFile(filename: "foo.c", directory: "/s/llvm/cmakebuild")
 !2 = !{}
-!3 = !{!4}
+!3 = !{!4, !8}
 !4 = !MDSubprogram(name: "debug_declare", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: false, function: void ()* @debug_declare, variables: !2)
 !5 = !MDSubroutineType(types: !6)
 !6 = !{null, !7}
 !7 = !MDBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
-!8 = !{i32 2, !"Dwarf Version", i32 4}
-!9 = !{i32 2, !"Debug Info Version", i32 3}
-!10 = !{!"clang version 3.7.0 (trunk 235150) (llvm/trunk 235152)"}
-!11 = !MDLocalVariable(tag: DW_TAG_arg_variable, name: "val", arg: 1, scope: !4, file: !1, line: 1, type: !7)
-!12 = !MDExpression()
-!13 = !MDLocation(line: 1, column: 24, scope: !4)
+!8 = !MDSubprogram(name: "debug_declare_morecasts", scope: !1, file: !1, line: 8, type: !5, isLocal: false, isDefinition: true, scopeLine: 8, flags: DIFlagPrototyped, isOptimized: false, function: void ()* @debug_declare_morecasts, variables: !2)
+!9 = !{i32 2, !"Dwarf Version", i32 4}
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = !{!"clang version 3.7.0 (trunk 235150) (llvm/trunk 235152)"}
+!12 = !MDLocalVariable(tag: DW_TAG_arg_variable, name: "val", arg: 1, scope: !4, file: !1, line: 1, type: !7)
+!13 = !MDExpression()
+!14 = !MDLocation(line: 1, column: 24, scope: !4)
+!15 = !MDLocalVariable(tag: DW_TAG_arg_variable, name: "var", arg: 1, scope: !8, file: !1, line: 9, type: !7)
+!16 = !MDLocation(line: 9, column: 24, scope: !8)