[llvm-bugs] [Bug 39765] New: missed optimization: simple for loop

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39765

            Bug ID: 39765
           Summary: missed optimization: simple for loop
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: superjoe30 at gmail.com
                CC: llvm-bugs at lists.llvm.org

=====================================
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.

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