[BPF] fix a bug in BPFMISimplifyPatchable pass with -O0

The recommended optimization level for BPF programs
is O2 since (1). BPF is running inside the kernel and
linux kernel won't work at -O0 level, and (2). Verifier
is not able to handle O0 code properly, e.g., potential
large stack size and a lot of spills.

But we should keep -O0 at least compiling.
This patch fixed a bug in BPFMISimplifyPatchable phase
where with -O0, a segmentation fault will happen for a
simple program like:
  int test(int a, int b) { return a + b; }

A test case is added to capture such a case.

Differential Revision: https://reviews.llvm.org/D73681

llvm/lib/Target/BPF/BPFMISimplifyPatchable.cpp

@@ -70,9 +70,10 @@ struct BPFMISimplifyPatchable : public MachineFunctionPass {
 public:
   // Main entry point for this pass.
   bool runOnMachineFunction(MachineFunction &MF) override {
-    if (!skipFunction(MF.getFunction())) {
-      initialize(MF);
-    }
+    if (skipFunction(MF.getFunction()))
+      return false;
+
+    initialize(MF);
     return removeLD();
   }
 };

llvm/test/CodeGen/BPF/optnone-1.ll

@@ -0,0 +1,52 @@
+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+; Source Code:
+;   int test(int a, int b) { return a + b; }
+; Compilation flag:
+;   clang -target bpf -g -S -emit-llvm test.c
+
+; Function Attrs: noinline nounwind optnone
+define dso_local i32 @test(i32 %a, i32 %b) #0 !dbg !7 {
+entry:
+  %a.addr = alloca i32, align 4
+  %b.addr = alloca i32, align 4
+  store i32 %a, i32* %a.addr, align 4
+  call void @llvm.dbg.declare(metadata i32* %a.addr, metadata !11, metadata !DIExpression()), !dbg !12
+  store i32 %b, i32* %b.addr, align 4
+  call void @llvm.dbg.declare(metadata i32* %b.addr, metadata !13, metadata !DIExpression()), !dbg !14
+  %0 = load i32, i32* %a.addr, align 4, !dbg !15
+  %1 = load i32, i32* %b.addr, align 4, !dbg !16
+  %add = add nsw i32 %0, %1, !dbg !17
+  ret i32 %add, !dbg !18
+}
+
+; CHECK-LABEL: test
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+attributes #0 = { noinline nounwind optnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind readnone speculatable}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.0 (https://github.com/llvm/llvm-project.git 7f49dc496682e38335b204bbae371a0ca5057719)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/llvm")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 11.0.0 (https://github.com/llvm/llvm-project.git 7f49dc496682e38335b204bbae371a0ca5057719)"}
+!7 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !10, !10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DILocalVariable(name: "a", arg: 1, scope: !7, file: !1, line: 1, type: !10)
+!12 = !DILocation(line: 1, column: 14, scope: !7)
+!13 = !DILocalVariable(name: "b", arg: 2, scope: !7, file: !1, line: 1, type: !10)
+!14 = !DILocation(line: 1, column: 21, scope: !7)
+!15 = !DILocation(line: 1, column: 33, scope: !7)
+!16 = !DILocation(line: 1, column: 37, scope: !7)
+!17 = !DILocation(line: 1, column: 35, scope: !7)
+!18 = !DILocation(line: 1, column: 26, scope: !7)