efriedma-quic
commented
5 years ago This is a limitation of ScalarEvolution, specifically ScalarEvolution::computeExitLimit. SimplifyCFG turns the comparisons into a switch, and computeExitLimit doesn't handle a switch with multiple values that exit the loop. Because ScalarEvolution can't compute the trip count, LoopDeletion is conservative and refuses to delete the loop. Probably easy to improve.
Extended Description
===================================== void entry(void) { for (unsigned i = 0; i != 10; i += 1) { if (i == 1) break; } }
expected output:
@entry with body deleted
actual output:
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128" target triple = "x86_64-unknown-linux-gnu"
define dso_local void @entry() local_unnamed_addr #0 !dbg !7 { call void @llvm.dbg.value(metadata i32 0, metadata !11, metadata !DIExpression()), !dbg !14 br label %1, !dbg !15
%2 = phi i32 [ 0, %0 ], [ %4, %3 ], !dbg !16 call void @llvm.dbg.value(metadata i32 %2, metadata !11, metadata !DIExpression()), !dbg !14 switch i32 %2, label %3 [ i32 10, label %5 i32 1, label %5 ], !dbg !17
%4 = add i32 %2, 1, !dbg !18 call void @llvm.dbg.value(metadata i32 %4, metadata !11, metadata !DIExpression()), !dbg !14 br label %1, !dbg !20, !llvm.loop !21
ret void, !dbg !23 }
declare void @llvm.dbg.value(metadata, metadata, metadata) #1
attributes #0 = { nounwind readnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-frame-pointer-elim"="false" "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" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "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 8.0.0 (trunk 346600)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None) !1 = !DIFile(filename: "/tmp/compiler-explorer-compiler1181023-55-yllx70.sg18/example.c", directory: "/tmp/compiler-explorer-compiler1181023-55-yllx70.sg18") !2 = !{} !3 = !{i32 2, !"Dwarf Version", i32 4} !4 = !{i32 2, !"Debug Info Version", i32 3} !5 = !{i32 1, !"wchar_size", i32 4} !6 = !{!"clang version 8.0.0 (trunk 346600)"} !7 = distinct !DISubprogram(name: "entry", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, retainedNodes: !10) !8 = !DISubroutineType(types: !9) !9 = !{null} !10 = !{#11} !11 = !DILocalVariable(name: "i", scope: !12, file: !1, line: 2, type: !13) !12 = distinct !DILexicalBlock(scope: !7, file: !1, line: 2, column: 5) !13 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned) !14 = !DILocation(line: 2, column: 19, scope: !12) !15 = !DILocation(line: 2, column: 10, scope: !12) !16 = !DILocation(line: 0, scope: !12) !17 = !DILocation(line: 2, column: 5, scope: !12) !18 = !DILocation(line: 2, column: 37, scope: !19) !19 = distinct !DILexicalBlock(scope: !12, file: !1, line: 2, column: 5) !20 = !DILocation(line: 2, column: 5, scope: !19) !21 = distinct !{#21, !17, !22} !22 = !DILocation(line: 4, column: 5, scope: !12) !23 = !DILocation(line: 5, column: 1, scope: !7)
hint: if you change it to i < 10 then the optimizer generates the desired output.