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BPF: avoid NE/EQ loop exit condition
Kuniyuki Iwashima reported in [1] that llvm compiler may convert a loop exit condition with "i < bound" to "i != bound", where "i" is the loop index variable and "bound" is the upper bound. In case that "bound" is not a constant, verifier will always have "i != bound" true, which will cause verifier failure since to verifier this is an infinite loop. The fix is to avoid transforming "i < bound" to "i != bound". In llvm, the transformation is done by IndVarSimplify pass. The compiler checks loop condition cost (i = i + 1) and if the cost is lower, it may transform "i < bound" to "i != bound". This patch implemented getArithmeticInstrCost() in BPF TargetTransformInfo class to return a higher cost for such an operation, which will prevent the transformation for the test case added in this patch. [1] https://lore.kernel.org/netdev/1994df05-8f01-371f-3c3b-d33d7836878c@fb.com/ Differential Revision: https://reviews.llvm.org/D107483 (cherry picked from commit e52946b9ababcbf8e6f40b1b15900ae2e795a1c6)
This commit is contained in:
Yonghong Song
Tom Stellard
parent
a558de374f
commit
f6a86e448a
@ -54,6 +54,24 @@ public:
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return BaseT::getCmpSelInstrCost(Opcode, ValTy, CondTy, VecPred, CostKind,
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I);
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}
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InstructionCost getArithmeticInstrCost(
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unsigned Opcode, Type *Ty,
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TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput,
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TTI::OperandValueKind Opd1Info = TTI::OK_AnyValue,
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TTI::OperandValueKind Opd2Info = TTI::OK_AnyValue,
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TTI::OperandValueProperties Opd1PropInfo = TTI::OP_None,
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TTI::OperandValueProperties Opd2PropInfo = TTI::OP_None,
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ArrayRef<const Value *> Args = ArrayRef<const Value *>(),
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const Instruction *CxtI = nullptr) {
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int ISD = TLI->InstructionOpcodeToISD(Opcode);
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if (ISD == ISD::ADD && CostKind == TTI::TCK_RecipThroughput)
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return SCEVCheapExpansionBudget.getValue() + 1;
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return BaseT::getArithmeticInstrCost(Opcode, Ty, CostKind, Opd1Info,
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Opd2Info, Opd1PropInfo,
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Opd2PropInfo);
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}
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};
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} // end namespace llvm
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131
test/CodeGen/BPF/loop-exit-cond.ll
Normal file
131
test/CodeGen/BPF/loop-exit-cond.ll
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@ -0,0 +1,131 @@
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; RUN: opt -O2 -S -o %t1 < %s
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; RUN: llc -march=bpf -mcpu=v3 %t1 -o - | FileCheck %s
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;
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; Source code:
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; typedef unsigned long u64;
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; void foo(char *data, int idx, u64 *);
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; int test(int len, char *data) {
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; if (len < 100) {
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; for (int i = 1; i < len; i++) {
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; u64 d[1];
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; d[0] = data[0] ?: '0';
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; foo("%c", i, d);
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; }
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; }
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; return 0;
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; }
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; Compilation flag:
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; clang -target bpf -O2 -S -emit-llvm -Xclang -disable-llvm-passes test.c
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; ModuleID = 'test.c'
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source_filename = "test.c"
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target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
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target triple = "bpf"
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@.str = private unnamed_addr constant [3 x i8] c"%c\00", align 1
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; Function Attrs: nounwind
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define dso_local i32 @test(i32 %len, i8* %data) #0 {
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entry:
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%len.addr = alloca i32, align 4
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%data.addr = alloca i8*, align 8
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%i = alloca i32, align 4
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%d = alloca [1 x i64], align 8
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store i32 %len, i32* %len.addr, align 4, !tbaa !3
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store i8* %data, i8** %data.addr, align 8, !tbaa !7
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%0 = load i32, i32* %len.addr, align 4, !tbaa !3
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%cmp = icmp slt i32 %0, 100
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br i1 %cmp, label %if.then, label %if.end
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if.then: ; preds = %entry
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%1 = bitcast i32* %i to i8*
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call void @llvm.lifetime.start.p0i8(i64 4, i8* %1) #3
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store i32 1, i32* %i, align 4, !tbaa !3
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br label %for.cond
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for.cond: ; preds = %for.inc, %if.then
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%2 = load i32, i32* %i, align 4, !tbaa !3
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%3 = load i32, i32* %len.addr, align 4, !tbaa !3
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%cmp1 = icmp slt i32 %2, %3
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br i1 %cmp1, label %for.body, label %for.cond.cleanup
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; CHECK: w[[LEN:[0-9]+]] = w1
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; CHECK: w[[IDX:[0-9]+]] += 1
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; CHECK-NEXT: w[[IDX]] s< w[[LEN]] goto
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for.cond.cleanup: ; preds = %for.cond
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%4 = bitcast i32* %i to i8*
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call void @llvm.lifetime.end.p0i8(i64 4, i8* %4) #3
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br label %for.end
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for.body: ; preds = %for.cond
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%5 = bitcast [1 x i64]* %d to i8*
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call void @llvm.lifetime.start.p0i8(i64 8, i8* %5) #3
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%6 = load i8*, i8** %data.addr, align 8, !tbaa !7
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%arrayidx = getelementptr inbounds i8, i8* %6, i64 0
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%7 = load i8, i8* %arrayidx, align 1, !tbaa !9
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%conv = sext i8 %7 to i32
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%tobool = icmp ne i32 %conv, 0
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br i1 %tobool, label %cond.true, label %cond.false
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cond.true: ; preds = %for.body
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br label %cond.end
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cond.false: ; preds = %for.body
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br label %cond.end
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cond.end: ; preds = %cond.false, %cond.true
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%cond = phi i32 [ %conv, %cond.true ], [ 48, %cond.false ]
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%conv2 = sext i32 %cond to i64
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%arrayidx3 = getelementptr inbounds [1 x i64], [1 x i64]* %d, i64 0, i64 0
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store i64 %conv2, i64* %arrayidx3, align 8, !tbaa !10
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%8 = load i32, i32* %i, align 4, !tbaa !3
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%arraydecay = getelementptr inbounds [1 x i64], [1 x i64]* %d, i64 0, i64 0
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call void @foo(i8* getelementptr inbounds ([3 x i8], [3 x i8]* @.str, i64 0, i64 0), i32 %8, i64* %arraydecay)
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%9 = bitcast [1 x i64]* %d to i8*
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call void @llvm.lifetime.end.p0i8(i64 8, i8* %9) #3
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br label %for.inc
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for.inc: ; preds = %cond.end
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%10 = load i32, i32* %i, align 4, !tbaa !3
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%inc = add nsw i32 %10, 1
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store i32 %inc, i32* %i, align 4, !tbaa !3
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br label %for.cond, !llvm.loop !12
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for.end: ; preds = %for.cond.cleanup
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br label %if.end
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if.end: ; preds = %for.end, %entry
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ret i32 0
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}
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; Function Attrs: argmemonly nofree nosync nounwind willreturn
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declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #1
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declare dso_local void @foo(i8*, i32, i64*) #2
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; Function Attrs: argmemonly nofree nosync nounwind willreturn
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declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #1
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attributes #0 = { nounwind "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
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attributes #1 = { argmemonly nofree nosync nounwind willreturn }
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attributes #2 = { "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
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attributes #3 = { nounwind }
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!llvm.module.flags = !{!0, !1}
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!llvm.ident = !{!2}
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!0 = !{i32 1, !"wchar_size", i32 4}
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!1 = !{i32 7, !"frame-pointer", i32 2}
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!2 = !{!"clang version 14.0.0 (https://github.com/llvm/llvm-project.git 8385de118443144518c9fba8b3d831d9076e746b)"}
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!3 = !{!4, !4, i64 0}
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!4 = !{!"int", !5, i64 0}
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!5 = !{!"omnipotent char", !6, i64 0}
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!6 = !{!"Simple C/C++ TBAA"}
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!7 = !{!8, !8, i64 0}
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!8 = !{!"any pointer", !5, i64 0}
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!9 = !{!5, !5, i64 0}
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!10 = !{!11, !11, i64 0}
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!11 = !{!"long", !5, i64 0}
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!12 = distinct !{!12, !13}
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!13 = !{!"llvm.loop.mustprogress"}
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