[ARM] Handle empty functions with debug info in load/store opt pass

[pbarrio]

Summary

In fuctions that contained debug info but were empty otherwise,
the ARM load/store optimizer could abort. This was because
function MergeReturnIntoLDM handled the special case where a
Machine Basic BLock is empty by calling MBB.empty(). However, this
returns false in presence of debug info, although the function
should be considered empty in the eyes of the load/store optimizer.
This has been fixed by handling the case where searching through the
block finds only debug instructions.

[pbarrio]

Hello Renato, Duncan, James,

Would you have a few minutes to review this simple fix? The test case is quite verbose, but that's the debug info to blame :)

I have reduced it as much as I could; please let me know if that could be done better.

Many thanks,
Pablo

[t.p.northover]

Just added llvm-commits as a subscriber so the mailing list gets notified.

[pbarrio]

Sorry and thanks!

[jmolloy]

Hi Pablo,

This LGTM.

Cheers,

James

lib/Target/ARM/ARMLoadStoreOptimizer.cpp

 /// =>
 ///   ldmfd sp!, {..., pc}
 bool ARMLoadStoreOpt::MergeReturnIntoLDM(MachineBasicBlock &MBB) {
   // Thumb1 LDM doesn't allow high registers.
   if (isThumb1) return false;
   if (MBB.empty()) return false;
 
   MachineBasicBlock::iterator MBBI = MBB.getLastNonDebugInstr();
-  if (MBBI != MBB.begin() &&
+  if (MBBI != MBB.begin() && MBBI != MBB.end() &&
       (MBBI->getOpcode() == ARM::BX_RET ||
        MBBI->getOpcode() == ARM::tBX_RET ||
        MBBI->getOpcode() == ARM::MOVPCLR)) {
     MachineBasicBlock::iterator PrevI = std::prev(MBBI);
     // Ignore any DBG_VALUE instructions.
     while (PrevI->isDebugValue() && PrevI != MBB.begin())
       --PrevI;
     MachineInstr &PrevMI = *PrevI;

test/CodeGen/ARM/2016-08-24-ARM-LDST-dbginfo-bug.ll

+; RUN: llc < %s -mtriple=thumbv7em-arm-none-eabi -O3
+
+; When using -Oz and -g, this code generated an abort in the ARM load/store optimizer.
+
+%struct.s = type { %struct.s* }
+
+; Function Attrs: minsize nounwind optsize readonly
+define %struct.s* @s_idx(%struct.s* readonly %xl) local_unnamed_addr #0 !dbg !8 {
+entry:
+  tail call void @llvm.dbg.value(metadata %struct.s* %xl, i64 0, metadata !17, metadata !18), !dbg !19
+  br label %while.cond, !dbg !20
+
+while.cond:                                       ; preds = %while.body, %entry
+  %xl.addr.0 = phi %struct.s* [ %xl, %entry ], [ %0, %while.body ]
+  %tobool = icmp eq %struct.s* %xl.addr.0, null
+  br i1 %tobool, label %while.end, label %while.body
+
+while.body:                                       ; preds = %while.cond
+  %next = getelementptr inbounds %struct.s, %struct.s* %xl.addr.0, i32 0, i32 0
+  %0 = load %struct.s*, %struct.s** %next, align 4
+  tail call void @llvm.dbg.value(metadata %struct.s* %0, i64 0, metadata !17, metadata !18), !dbg !19
+  br label %while.cond
+
+while.end:                                        ; preds = %while.cond
+  ret %struct.s* null
+}
+
+; Function Attrs: nounwind readnone
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata) #1
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 4.0.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "test.c", directory: "/a/b/c")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{i32 1, !"min_enum_size", i32 4}
+!7 = !{!"clang version 4.0.0 "}
+!8 = distinct !DISubprogram(name: "s_idx", scope: !1, file: !1, line: 6, type: !9, isLocal: false, isDefinition: true, scopeLine: 7, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !16)
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11, !11}
+!11 = !DIDerivedType(tag: DW_TAG_typedef, name: "ezxml_t", file: !1, line: 1, baseType: !12)
+!12 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !13, size: 32, align: 32)
+!13 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "s", file: !1, line: 2, size: 32, align: 32, elements: !14)
+!14 = !{!15}
+!15 = !DIDerivedType(tag: DW_TAG_member, name: "next", scope: !13, file: !1, line: 3, baseType: !11, size: 32, align: 32)
+!16 = !{!17}
+!17 = !DILocalVariable(name: "xl", arg: 1, scope: !8, file: !1, line: 6, type: !11)
+!18 = !DIExpression()
+!19 = !DILocation(line: 6, column: 27, scope: !8)
+!20 = !DILocation(line: 8, column: 5, scope: !8)