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[BPF] Reset machine register kill mark in BPFMISimplifyPatchable
When LLVM is build with `LLVM_ENABLE_EXPENSIVE_CHECKS=ON` option the following C code snippet: struct t { unsigned long a; } __attribute__((preserve_access_index)); void foo(volatile struct t *t, volatile unsigned long *p) { *p = t->a; *p = t->a; } Causes an assertion: $ clang -g -O2 -c --target=bpf -mcpu=v2 t2.c -o /dev/null # After BPF PreEmit SimplifyPatchable # Machine code for function foo: IsSSA, TracksLiveness Function Live Ins: $r1 in %0, $r2 in %1 bb.0.entry: liveins: $r1, $r2 DBG_VALUE $r1, $noreg, !"t", !DIExpression() DBG_VALUE $r2, $noreg, !"p", !DIExpression() %1:gpr = COPY $r2 DBG_VALUE %1:gpr, $noreg, !"p", !DIExpression() %0:gpr = COPY $r1 DBG_VALUE %0:gpr, $noreg, !"t", !DIExpression() %2:gpr = LD_imm64 @"llvm.t:0:0$0:0" %4:gpr = ADD_rr %0:gpr(tied-def 0), killed %2:gpr %5:gpr = CORE_LD 344, %0:gpr, @"llvm.t:0:0$0:0" STD killed %5:gpr, %1:gpr, 0 %7:gpr = ADD_rr %0:gpr(tied-def 0), killed %2:gpr %8:gpr = CORE_LD 344, %0:gpr, @"llvm.t:0:0$0:0" STD killed %8:gpr, %1:gpr, 0 RET # End machine code for function foo. *** Bad machine code: Using a killed virtual register *** - function: foo - basic block: %bb.0 entry (0x6210000e6690) - instruction: %7:gpr = ADD_rr %0:gpr(tied-def 0), killed %2:gpr - operand 2: killed %2:gpr This happens because of the way BPFMISimplifyPatchable::processDstReg() updates second operand of the `ADD_rr` instruction. Code before `BPFMISimplifyPatchable`: .-> %2:gpr = LD_imm64 @"llvm.t:0:0$0:0" | |`----------------. | %3:gpr = LDD %2:gpr, 0 | %4:gpr = ADD_rr %0:gpr(tied-def 0), killed %3:gpr <--- (1) | %5:gpr = LDD killed %4:gpr, 0 ^^^^^^^^^^^^^ | STD killed %5:gpr, %1:gpr, 0 this is updated `----------------. %6:gpr = LDD %2:gpr, 0 %7:gpr = ADD_rr %0:gpr(tied-def 0), killed %6:gpr <--- (2) %8:gpr = LDD killed %7:gpr, 0 ^^^^^^^^^^^^^ STD killed %8:gpr, %1:gpr, 0 this is updated Instructions (1) and (2) would be updated to: ADD_rr %0:gpr(tied-def 0), killed %2:gpr The `killed` mark is inherited from machine operands `killed %3:gpr` and `killed %6:gpr` which are updated inplace by `processDstReg()`. This commit updates `processDstReg()` reset kill marks for updated machine operands to keep liveness information conservatively correct. Differential Revision: https://reviews.llvm.org/D157805 (cherry picked from commit 27026fe)
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‎llvm/lib/Target/BPF/BPFMISimplifyPatchable.cpp‎

Lines changed: 25 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -207,8 +207,32 @@ void BPFMISimplifyPatchable::processDstReg(MachineRegisterInfo *MRI,
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decltype(End) NextI;
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for (auto I = Begin; I != End; I = NextI) {
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NextI = std::next(I);
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if (doSrcRegProp)
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if (doSrcRegProp) {
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// In situations like below it is not known if usage is a kill
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// after setReg():
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//
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// .-> %2:gpr = LD_imm64 @"llvm.t:0:0$0:0"
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// |
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// |`----------------.
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// | %3:gpr = LDD %2:gpr, 0
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// | %4:gpr = ADD_rr %0:gpr(tied-def 0), killed %3:gpr <--- (1)
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// | %5:gpr = LDD killed %4:gpr, 0 ^^^^^^^^^^^^^
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// | STD killed %5:gpr, %1:gpr, 0 this is I
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// `----------------.
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// %6:gpr = LDD %2:gpr, 0
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// %7:gpr = ADD_rr %0:gpr(tied-def 0), killed %6:gpr <--- (2)
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// %8:gpr = LDD killed %7:gpr, 0 ^^^^^^^^^^^^^
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// STD killed %8:gpr, %1:gpr, 0 this is I
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//
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// Instructions (1) and (2) would be updated by setReg() to:
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//
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// ADD_rr %0:gpr(tied-def 0), %2:gpr
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//
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// %2:gpr is not killed at (1), so it is necessary to remove kill flag
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// from I.
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I->setReg(SrcReg);
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I->setIsKill(false);
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}
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// The candidate needs to have a unique definition.
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if (IsAma && MRI->getUniqueVRegDef(I->getReg()))

‎llvm/test/CodeGen/BPF/CORE/simplify-patchable-liveness-bug.ll‎

Lines changed: 121 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,121 @@
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; RUN: llc -mtriple=bpf -mcpu=v2 < %s | FileCheck -check-prefixes=CHECK %s
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; Test for machine register liveness update bug in
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; BPFMISimplifyPatchable::processDstReg.
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;
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; Generated from the following source code:
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; struct t {
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; unsigned long a;
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; } __attribute__((preserve_access_index));
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;
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; void foo(volatile struct t *t, volatile unsigned long *p) {
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; *p = t->a;
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; *p = t->a;
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; }
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;
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; Using the following command:
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; clang -g -O2 -S -emit-llvm --target=bpf t.c -o t.ll
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@"llvm.t:0:0$0:0" = external global i64, !llvm.preserve.access.index !0 #0
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; Function Attrs: nofree nounwind
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define dso_local void @foo(ptr noundef %t, ptr noundef %p) local_unnamed_addr #1 !dbg !12 {
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entry:
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call void @llvm.dbg.value(metadata ptr %t, metadata !20, metadata !DIExpression()), !dbg !22
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call void @llvm.dbg.value(metadata ptr %p, metadata !21, metadata !DIExpression()), !dbg !22
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%0 = load i64, ptr @"llvm.t:0:0$0:0", align 8
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%1 = getelementptr i8, ptr %t, i64 %0
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%2 = tail call ptr @llvm.bpf.passthrough.p0.p0(i32 0, ptr %1)
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%3 = load volatile i64, ptr %2, align 8, !dbg !23, !tbaa !24
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store volatile i64 %3, ptr %p, align 8, !dbg !29, !tbaa !30
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%4 = load i64, ptr @"llvm.t:0:0$0:0", align 8
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%5 = getelementptr i8, ptr %t, i64 %4
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%6 = tail call ptr @llvm.bpf.passthrough.p0.p0(i32 1, ptr %5)
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%7 = load volatile i64, ptr %6, align 8, !dbg !31, !tbaa !24
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store volatile i64 %7, ptr %p, align 8, !dbg !32, !tbaa !30
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ret void, !dbg !33
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}
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; CHECK: foo:
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; CHECK: prologue_end
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; CHECK-NEXT: .Ltmp[[LABEL1:.*]]:
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; CHECK-NEXT: .Ltmp
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; CHECK-NEXT: r[[#A:]] = *(u64 *)(r1 + 0)
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; CHECK-NEXT: .loc
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; CHECK-NEXT: .Ltmp
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; CHECK-NEXT: *(u64 *)(r2 + 0) = r[[#A]]
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; CHECK-NEXT: .loc
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; CHECK-NEXT: .Ltmp[[LABEL2:.*]]:
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; CHECK-NEXT: .Ltmp
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; CHECK-NEXT: r[[#B:]] = *(u64 *)(r1 + 0)
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; CHECK-NEXT: .Ltmp
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; CHECK-NEXT: .Ltmp
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; CHECK-NEXT: .loc
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; CHECK-NEXT: .Ltmp
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; CHECK-NEXT: *(u64 *)(r2 + 0) = r[[#B]]
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; CHECK: .section .BTF
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; CHECK: .long [[STR_T:.*]] # BTF_KIND_STRUCT(id = [[ID:.*]])
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; CHECK: .byte 116 # string offset=[[STR_T]]
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; CHECK: .ascii "0:0" # string offset=[[STR_A:.*]]
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; CHECK: # FieldReloc
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; CHECK: .long .Ltmp[[LABEL1]]
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; CHECK-NEXT: .long [[ID]]
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; CHECK-NEXT: .long [[STR_A]]
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; CHECK-NEXT: .long 0
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; CHECK: .long .Ltmp[[LABEL2]]
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; CHECK-NEXT: .long [[ID]]
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; CHECK-NEXT: .long [[STR_A]]
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; CHECK-NEXT: .long 0
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; Function Attrs: nofree nosync nounwind memory(none)
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declare ptr @llvm.bpf.passthrough.p0.p0(i32, ptr) #2
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; Function Attrs: nocallback nofree nosync nounwind speculatable willreturn memory(none)
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declare void @llvm.dbg.value(metadata, metadata, metadata) #3
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attributes #0 = { "btf_ama" }
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attributes #1 = { nofree nounwind "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
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attributes #2 = { nofree nosync nounwind memory(none) }
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attributes #3 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }
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!llvm.dbg.cu = !{!5}
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!llvm.module.flags = !{!6, !7, !8, !9, !10}
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!llvm.ident = !{!11}
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!0 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t", file: !1, line: 1, size: 64, elements: !2)
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!1 = !DIFile(filename: "some.file", directory: "/some/dir", checksumkind: CSK_MD5, checksum: "a149cfaf65a83125e7f2b2f47e5c7287")
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!2 = !{!3}
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!3 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !0, file: !1, line: 2, baseType: !4, size: 64)
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!4 = !DIBasicType(name: "unsigned long", size: 64, encoding: DW_ATE_unsigned)
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!5 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 18.0.0 (/home/eddy/work/llvm-project/clang 3810f2eb4382d5e2090ce5cd47f45379cb453c35)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
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!6 = !{i32 7, !"Dwarf Version", i32 5}
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!7 = !{i32 2, !"Debug Info Version", i32 3}
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!8 = !{i32 1, !"wchar_size", i32 4}
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!9 = !{i32 7, !"frame-pointer", i32 2}
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!10 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
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!11 = !{!"clang version 18.0.0 (/home/eddy/work/llvm-project/clang 3810f2eb4382d5e2090ce5cd47f45379cb453c35)"}
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!12 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 4, type: !13, scopeLine: 4, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !5, retainedNodes: !19)
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!13 = !DISubroutineType(types: !14)
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!14 = !{null, !15, !17}
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!15 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !16, size: 64)
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!16 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !0)
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!17 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !18, size: 64)
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!18 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !4)
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!19 = !{!20, !21}
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!20 = !DILocalVariable(name: "t", arg: 1, scope: !12, file: !1, line: 4, type: !15)
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!21 = !DILocalVariable(name: "p", arg: 2, scope: !12, file: !1, line: 4, type: !17)
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!22 = !DILocation(line: 0, scope: !12)
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!23 = !DILocation(line: 5, column: 11, scope: !12)
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!24 = !{!25, !26, i64 0}
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!25 = !{!"t", !26, i64 0}
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!26 = !{!"long", !27, i64 0}
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!27 = !{!"omnipotent char", !28, i64 0}
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!28 = !{!"Simple C/C++ TBAA"}
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!29 = !DILocation(line: 5, column: 6, scope: !12)
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!30 = !{!26, !26, i64 0}
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!31 = !DILocation(line: 6, column: 11, scope: !12)
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!32 = !DILocation(line: 6, column: 6, scope: !12)
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!33 = !DILocation(line: 7, column: 1, scope: !12)

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