[mips] Fix analyzeBranch to handle debug data

[sdardis]

Summary

In the case where there was a conditional branch followed by a unconditional
branch with debug instruction separating them, MipsInstrInfo::analyzeBranch
would not skip past debug instruction when searching for the second branch
which give erroneous results about the control flow of the block.

This could lead to the branch folder to merge the non-fall through case
into it's predecessor, leaving the conditional branch with a dangling
basic block operand.

This resolves PR34975.

Thanks to Alexander Richardson for reporting the issue!

[arichardson]

With this patch applied I can successfully compile the source file that was crashing. Thanks!

[atanasyan]

LGTM

llvm/trunk/lib/Target/Mips/MipsInstrInfo.cpp

MipsInstrInfo::BranchType MipsInstrInfo::analyzeBranch(
   // Not an analyzable branch (e.g., indirect jump).
   if (!getAnalyzableBrOpc(LastOpc))
     return LastInst->isIndirectBranch() ? BT_Indirect : BT_None;
 
   // Get the second to last instruction in the block.
   unsigned SecondLastOpc = 0;
   MachineInstr *SecondLastInst = nullptr;
 
-  if (++I != REnd) {
+  // Skip past any debug instruction to see if the second last actual
+  // is a branch.
+  ++I;
+  while (I != REnd && I->isDebugValue())
+    ++I;
+
+  if (I != REnd) {
     SecondLastInst = &*I;
     SecondLastOpc = getAnalyzableBrOpc(SecondLastInst->getOpcode());
 
     // Not an analyzable branch (must be an indirect jump).
     if (isUnpredicatedTerminator(*SecondLastInst) && !SecondLastOpc)
       return BT_None;
   }
 

llvm/trunk/test/CodeGen/Mips/pr34975.ll

+; RUN: llc -mtriple=mips64-unknown-freebsd -target-abi n64 -relocation-model pic -o /dev/null %s -O2
+
+; Test that the presence of debug information does not cause the branch folder
+; to rewrite branches to have negative basic block ids, which would cause the
+; long branch pass to crash.
+
+@c = external global i32, align 4
+
+define void @e() !dbg !19 {
+entry:
+  %0 = load i32, i32* @c, align 4, !dbg !28, !tbaa !31
+  %tobool8 = icmp eq i32 %0, 0, !dbg !35
+  br i1 %tobool8, label %for.end, label %for.body.preheader, !dbg !35
+
+for.body.preheader:                               ; preds = %entry
+  br label %for.body, !dbg !36
+
+for.body:                                         ; preds = %for.body.preheader
+  %1 = load i8, i8* undef, align 1, !dbg !36, !tbaa !38
+  %conv = zext i8 %1 to i32, !dbg !36
+  %cmp = icmp sgt i32 %0, %conv, !dbg !39
+  br i1 %cmp, label %if.end, label %if.then, !dbg !40
+
+if.then:                                          ; preds = %for.body
+  tail call void @llvm.dbg.value(metadata i32 %conv, metadata !41, metadata !DIExpression()), !dbg !43
+  %idxprom5 = zext i8 %1 to i64, !dbg !44
+  %call = tail call i32 bitcast (i32 (...)* @g to i32 (i32)*)(i32 signext undef) #3, !dbg !45
+  br label %if.end, !dbg !46
+
+if.end:                                           ; preds = %if.then, %for.body
+  unreachable
+
+for.end:                                          ; preds = %entry
+  ret void
+}
+
+declare i32 @g(...)
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!17, !18}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 6.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !3)
+!1 = !DIFile(filename: "/local/scratch/alr48/cheri/llvm/tools/clang/test/CodeGen/<stdin>", directory: "/local/scratch/alr48/cheri/llvm/cmake-build-debug/tools/clang/test/CodeGen")
+!2 = !{}
+!3 = !{!4, !9, !13, !15}
+!4 = !DIGlobalVariableExpression(var: !5, expr: !DIExpression())
+!5 = distinct !DIGlobalVariable(name: "a", scope: !0, file: !6, line: 6, type: !7, isLocal: false, isDefinition: true)
+!6 = !DIFile(filename: "/crash.c", directory: "/tmp")
+!7 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !8, size: 64)
+!8 = !DIBasicType(name: "unsigned char", size: 8, encoding: DW_ATE_unsigned_char)
+!9 = !DIGlobalVariableExpression(var: !10, expr: !DIExpression())
+!10 = distinct !DIGlobalVariable(name: "b", scope: !0, file: !6, line: 7, type: !11, isLocal: false, isDefinition: true)
+!11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !DIGlobalVariableExpression(var: !14, expr: !DIExpression())
+!14 = distinct !DIGlobalVariable(name: "c", scope: !0, file: !6, line: 8, type: !12, isLocal: false, isDefinition: true)
+!15 = !DIGlobalVariableExpression(var: !16, expr: !DIExpression())
+!16 = distinct !DIGlobalVariable(name: "d", scope: !0, file: !6, line: 8, type: !12, isLocal: false, isDefinition: true)
+!17 = !{i32 2, !"Debug Info Version", i32 3}
+!18 = !{i32 7, !"PIC Level", i32 2}
+!19 = distinct !DISubprogram(name: "e", scope: !6, file: !6, line: 9, type: !20, isLocal: false, isDefinition: true, scopeLine: 9, isOptimized: true, unit: !0, variables: !22)
+!20 = !DISubroutineType(types: !21)
+!21 = !{!12}
+!22 = !{!23}
+!23 = !DILocalVariable(name: "f", scope: !24, file: !6, line: 12, type: !12)
+!24 = distinct !DILexicalBlock(scope: !25, file: !6, line: 11, column: 20)
+!25 = distinct !DILexicalBlock(scope: !26, file: !6, line: 11, column: 9)
+!26 = distinct !DILexicalBlock(scope: !27, file: !6, line: 10, column: 3)
+!27 = distinct !DILexicalBlock(scope: !19, file: !6, line: 10, column: 3)
+!28 = !DILocation(line: 10, column: 10, scope: !29)
+!29 = distinct !DILexicalBlock(scope: !30, file: !6, line: 10, column: 3)
+!30 = distinct !DILexicalBlock(scope: !19, file: !6, line: 10, column: 3)
+!31 = !{!32, !32, i64 0}
+!32 = !{!"int", !33, i64 0}
+!33 = !{!"omnipotent char", !34, i64 0}
+!34 = !{!"Simple C/C++ TBAA"}
+!35 = !DILocation(line: 10, column: 3, scope: !30)
+!36 = !DILocation(line: 11, column: 9, scope: !37)
+!37 = distinct !DILexicalBlock(scope: !29, file: !6, line: 11, column: 9)
+!38 = !{!33, !33, i64 0}
+!39 = !DILocation(line: 11, column: 14, scope: !37)
+!40 = !DILocation(line: 11, column: 9, scope: !29)
+!41 = !DILocalVariable(name: "f", scope: !42, file: !6, line: 12, type: !12)
+!42 = distinct !DILexicalBlock(scope: !37, file: !6, line: 11, column: 20)
+!43 = !DILocation(line: 12, column: 11, scope: !42)
+!44 = !DILocation(line: 13, column: 9, scope: !42)
+!45 = !DILocation(line: 13, column: 7, scope: !42)
+!46 = !DILocation(line: 14, column: 5, scope: !42)