[BPF] disable ReduceLoadWidth during SelectionDag phase

[yonghong-song]

Summary

The compiler may transform the following code
(e.g., the test case in this commit)

ctx = ctx + reloc_offset
... (*(u32 *)ctx) & 0x8000 ...

to

ctx = ctx + reloc_offset
... (*(u8 *)(ctx + 1)) & 0x80 ...

where reloc_offset will be replaced with a constant during
AsmPrinter phase.

The above transformed code will be rejected the kernel verifier
as it does not allow

*(type *)((ctx + non_zero_offset1) + non_zero_offset2)

style access pattern.

It is hard at SelectionDag phase to identify whether a load
is related to context or not. Sometime, interprocedure analysis
may be needed. So let us simply prevent such optimization
from happening.

[anakryiko]

looks simple enough, thanks for fixing it fast!

llvm/lib/Target/BPF/BPFISelLowering.h

EVT getOptimalMemOpType(const MemOp &Op,
     return Op.size() >= 8 ? MVT::i64 : MVT::i32;
   }
 
   bool shouldConvertConstantLoadToIntImm(const APInt &Imm,
                                          Type *Ty) const override {
     return true;
   }
 
+  // Prevent reducing load width during SelectionDag phase.
+  // Otherwise, we may transform the following
+  //   ctx = ctx + reloc_offset
+  //   ... (*(u32 *)ctx) & 0x8000...
+  // to
+  //   ctx = ctx + reloc_offset
+  //   ... (*(u8 *)(ctx + 1)) & 0x80 ...
+  // which will be rejected by the verifier.
+  bool shouldReduceLoadWidth(SDNode *Load, ISD::LoadExtType ExtTy,
+                             EVT NewVT) const override {
+    return false;
+  }
+
   unsigned EmitSubregExt(MachineInstr &MI, MachineBasicBlock *BB, unsigned Reg,
                          bool isSigned) const;
 
   MachineBasicBlock * EmitInstrWithCustomInserterMemcpy(MachineInstr &MI,
                                                         MachineBasicBlock *BB)
                                                         const;
 
 };
 }
 
 #endif

llvm/test/CodeGen/BPF/CORE/no-narrow-load.ll

+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfeb -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+; Source code:
+;   struct data_t {
+;     int d1;
+;     int d2;
+;   };
+;   struct info_t {
+;     int pid;
+;     int flags;
+;   } __attribute__((preserve_access_index));
+;
+;   extern void output(void *);
+;   void test(struct info_t * args) {
+;     int is_mask2 = args->flags & 0x10000;
+;     struct data_t data = {};
+;
+;     data.d1 = is_mask2 ? 2 : args->pid;
+;     data.d2 = (is_mask2 || (args->flags & 0x8000)) ? 1 : 2;
+;     output(&data);
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm test.c
+
+%struct.info_t = type { i32, i32 }
+%struct.data_t = type { i32, i32 }
+
+; Function Attrs: nounwind
+define dso_local void @test(%struct.info_t* readonly %args) local_unnamed_addr #0 !dbg !12 {
+entry:
+  %data = alloca i64, align 8
+  %tmpcast = bitcast i64* %data to %struct.data_t*
+  call void @llvm.dbg.value(metadata %struct.info_t* %args, metadata !22, metadata !DIExpression()), !dbg !29
+  %0 = tail call i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.info_ts(%struct.info_t* %args, i32 1, i32 1), !dbg !30, !llvm.preserve.access.index !16
+  %1 = load i32, i32* %0, align 4, !dbg !30, !tbaa !31
+  %and = and i32 %1, 65536, !dbg !36
+  call void @llvm.dbg.value(metadata i32 %and, metadata !23, metadata !DIExpression()), !dbg !29
+  %2 = bitcast i64* %data to i8*, !dbg !37
+  call void @llvm.lifetime.start.p0i8(i64 8, i8* nonnull %2) #5, !dbg !37
+  call void @llvm.dbg.declare(metadata %struct.data_t* %tmpcast, metadata !24, metadata !DIExpression()), !dbg !38
+  store i64 0, i64* %data, align 8, !dbg !38
+  %tobool = icmp eq i32 %and, 0, !dbg !39
+  br i1 %tobool, label %cond.false, label %lor.end.critedge, !dbg !39
+
+cond.false:                                       ; preds = %entry
+  %3 = tail call i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.info_ts(%struct.info_t* %args, i32 0, i32 0), !dbg !40, !llvm.preserve.access.index !16
+  %4 = load i32, i32* %3, align 4, !dbg !40, !tbaa !41
+  %d1 = bitcast i64* %data to i32*, !dbg !42
+  store i32 %4, i32* %d1, align 8, !dbg !43, !tbaa !44
+  %5 = load i32, i32* %0, align 4, !dbg !46, !tbaa !31
+  %and2 = and i32 %5, 32768, !dbg !47
+  %tobool3 = icmp eq i32 %and2, 0, !dbg !48
+  %phitmp = select i1 %tobool3, i32 2, i32 1, !dbg !48
+  br label %lor.end, !dbg !48
+
+lor.end.critedge:                                 ; preds = %entry
+  %d1.c = bitcast i64* %data to i32*, !dbg !42
+  store i32 2, i32* %d1.c, align 8, !dbg !43, !tbaa !44
+  br label %lor.end, !dbg !48
+
+lor.end:                                          ; preds = %lor.end.critedge, %cond.false
+  %6 = phi i32 [ %phitmp, %cond.false ], [ 1, %lor.end.critedge ]
+  %d2 = getelementptr inbounds %struct.data_t, %struct.data_t* %tmpcast, i64 0, i32 1, !dbg !49
+  store i32 %6, i32* %d2, align 4, !dbg !50, !tbaa !51
+  call void @output(i8* nonnull %2) #5, !dbg !52
+  call void @llvm.lifetime.end.p0i8(i64 8, i8* nonnull %2) #5, !dbg !53
+  ret void, !dbg !53
+}
+
+; CHECK: r[[LOAD1:[0-9]+]] = *(u32 *)(r{{[0-9]+}} + 4)
+; CHECK: r[[LOAD1]] &= 65536
+; CHECK: r[[LOAD2:[0-9]+]] = *(u32 *)(r{{[0-9]+}} + 4)
+; CHECK: r[[LOAD2]] &= 32768
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #2
+
+; Function Attrs: nounwind readnone
+declare i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.info_ts(%struct.info_t*, i32 immarg, i32 immarg) #3
+
+declare !dbg !4 dso_local void @output(i8*) local_unnamed_addr #4
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #2
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { nounwind "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 willreturn }
+attributes #2 = { argmemonly nounwind willreturn }
+attributes #3 = { nounwind readnone }
+attributes #4 = { "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "no-infs-fp-math"="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 #5 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!8, !9, !10}
+!llvm.ident = !{!11}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.0 (https://github.com/llvm/llvm-project.git 5884aae58f56786475bbc0f13ad8bd35f7f1ce69)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/core")
+!2 = !{}
+!3 = !{!4}
+!4 = !DISubprogram(name: "output", scope: !1, file: !1, line: 10, type: !5, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
+!5 = !DISubroutineType(types: !6)
+!6 = !{null, !7}
+!7 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
+!8 = !{i32 7, !"Dwarf Version", i32 4}
+!9 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{i32 1, !"wchar_size", i32 4}
+!11 = !{!"clang version 11.0.0 (https://github.com/llvm/llvm-project.git 5884aae58f56786475bbc0f13ad8bd35f7f1ce69)"}
+!12 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 11, type: !13, scopeLine: 11, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !21)
+!13 = !DISubroutineType(types: !14)
+!14 = !{null, !15}
+!15 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !16, size: 64)
+!16 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "info_t", file: !1, line: 5, size: 64, elements: !17)
+!17 = !{!18, !20}
+!18 = !DIDerivedType(tag: DW_TAG_member, name: "pid", scope: !16, file: !1, line: 6, baseType: !19, size: 32)
+!19 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!20 = !DIDerivedType(tag: DW_TAG_member, name: "flags", scope: !16, file: !1, line: 7, baseType: !19, size: 32, offset: 32)
+!21 = !{!22, !23, !24}
+!22 = !DILocalVariable(name: "args", arg: 1, scope: !12, file: !1, line: 11, type: !15)
+!23 = !DILocalVariable(name: "is_mask2", scope: !12, file: !1, line: 12, type: !19)
+!24 = !DILocalVariable(name: "data", scope: !12, file: !1, line: 13, type: !25)
+!25 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "data_t", file: !1, line: 1, size: 64, elements: !26)
+!26 = !{!27, !28}
+!27 = !DIDerivedType(tag: DW_TAG_member, name: "d1", scope: !25, file: !1, line: 2, baseType: !19, size: 32)
+!28 = !DIDerivedType(tag: DW_TAG_member, name: "d2", scope: !25, file: !1, line: 3, baseType: !19, size: 32, offset: 32)
+!29 = !DILocation(line: 0, scope: !12)
+!30 = !DILocation(line: 12, column: 24, scope: !12)
+!31 = !{!32, !33, i64 4}
+!32 = !{!"info_t", !33, i64 0, !33, i64 4}
+!33 = !{!"int", !34, i64 0}
+!34 = !{!"omnipotent char", !35, i64 0}
+!35 = !{!"Simple C/C++ TBAA"}
+!36 = !DILocation(line: 12, column: 30, scope: !12)
+!37 = !DILocation(line: 13, column: 3, scope: !12)
+!38 = !DILocation(line: 13, column: 17, scope: !12)
+!39 = !DILocation(line: 15, column: 13, scope: !12)
+!40 = !DILocation(line: 15, column: 34, scope: !12)
+!41 = !{!32, !33, i64 0}
+!42 = !DILocation(line: 15, column: 8, scope: !12)
+!43 = !DILocation(line: 15, column: 11, scope: !12)
+!44 = !{!45, !33, i64 0}
+!45 = !{!"data_t", !33, i64 0, !33, i64 4}
+!46 = !DILocation(line: 16, column: 33, scope: !12)
+!47 = !DILocation(line: 16, column: 39, scope: !12)
+!48 = !DILocation(line: 16, column: 23, scope: !12)
+!49 = !DILocation(line: 16, column: 8, scope: !12)
+!50 = !DILocation(line: 16, column: 11, scope: !12)
+!51 = !{!45, !33, i64 4}
+!52 = !DILocation(line: 17, column: 3, scope: !12)
+!53 = !DILocation(line: 18, column: 1, scope: !12)