[llvm-dev] is this a bug in an optimization pass?

[Andrew Kelley via llvm-dev]

The frontend code is a pretty simple for loop, that counts from i = 0;
i != 10; i += 1
It gets optimized into and endless loop.

export fn entry() void {
    var array: [10]Bar = undefined;
    var x = for (array) |elem, i| {
        if (i == 1) break elem;
    } else bar2();
}

Here's the generated IR:

; ModuleID = 'test'
source_filename = "test"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

%Bar = type { i32, i32 }
%"[]u8" = type { i8*, i64 }
%StackTrace = type { i64, %"[]usize" }
%"[]usize" = type { i64*, i64 }

@0 = internal unnamed_addr constant %Bar { i32 3, i32 4 }, align 4

; Function Attrs: nounwind readnone speculatable
declare void @llvm.dbg.declare(metadata, metadata, metadata) #1

; Function Attrs: nobuiltin nounwind
define void @entry() #2 !dbg !41 {
Entry:
  %array = alloca [10 x %Bar], align 4
  %x = alloca %Bar, align 4
  %for_index = alloca i64, align 8
  %0 = bitcast [10 x %Bar]* %array to i8*, !dbg !60
  call void @llvm.memset.p0i8.i64(i8* align 4 %0, i8 -86, i64 80, i1
false), !dbg !60
  call void @llvm.dbg.declare(metadata [10 x %Bar]* %array, metadata
!45, metadata !DIExpression()), !dbg !60
  store i64 0, i64* %for_index, align 8, !dbg !61
  call void @llvm.dbg.declare(metadata i64* %for_index, metadata !55,
metadata !DIExpression()), !dbg !63
  br label %ForCond, !dbg !61

ForCond:                                          ; preds = %Else, %Entry
  %1 = load i64, i64* %for_index, align 8, !dbg !61
  %2 = icmp ne i64 %1, 10, !dbg !61
  br i1 %2, label %ForBody, label %ForElse, !dbg !61

ForBody:                                          ; preds = %ForCond
  %3 = getelementptr inbounds [10 x %Bar], [10 x %Bar]* %array, i64 0,
i64 %1, !dbg !61
  call void @llvm.dbg.declare(metadata %Bar* %3, metadata !58,
metadata !DIExpression()), !dbg !64
  %4 = load i64, i64* %for_index, align 8, !dbg !65
  %5 = icmp eq i64 %4, 1, !dbg !67
  br i1 %5, label %Then, label %Else, !dbg !67

Then:                                             ; preds = %ForBody
  %6 = bitcast %Bar* %3 to i8*, !dbg !68
  %7 = bitcast %Bar* %x to i8*, !dbg !68
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %7, i8* align 4 %6,
i64 8, i1 false), !dbg !68
  br label %ForEnd, !dbg !69

Else:                                             ; preds = %ForBody
  %8 = add nuw i64 %1, 1, !dbg !61
  store i64 %8, i64* %for_index, align 8, !dbg !61
  br label %ForCond, !dbg !61

ForElse:                                          ; preds = %ForCond
  call fastcc void @bar2(%Bar* sret %x), !dbg !70
  br label %ForEnd, !dbg !71

ForEnd:                                           ; preds = %ForElse, %Then
  call void @llvm.dbg.declare(metadata %Bar* %x, metadata !59,
metadata !DIExpression()), !dbg !72
  ret void, !dbg !73
}

; Function Attrs: argmemonly nounwind
declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1) #3

; Function Attrs: argmemonly nounwind
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8*
nocapture readonly, i64, i1) #3

; Function Attrs: nobuiltin nounwind
define internal fastcc void @bar2(%Bar* nonnull sret) unnamed_addr #2 !dbg !74 {
Entry:
  call fastcc void @bar(%Bar* sret %0), !dbg !79
  ret void, !dbg !81
}

; Function Attrs: nobuiltin nounwind
define internal fastcc void @bar(%Bar* nonnull sret) unnamed_addr #2 !dbg !82 {
Entry:
  %1 = bitcast %Bar* %0 to i8*, !dbg !83
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %1, i8* align 4
bitcast (%Bar* @0 to i8*), i64 8, i1 false), !dbg !83
  ret void, !dbg !83
}


Here's the optimized IR, an endless loop:

define void @entry() local_unnamed_addr #0 !dbg !8 {
Entry:
  call void @llvm.dbg.value(metadata i64 0, metadata !24, metadata
!DIExpression()), !dbg !30
  br label %ForCond, !dbg !31

ForCond:                                          ; preds = %Else, %Entry
  %for_index.0 = phi i64 [ 0, %Entry ], [ %0, %Else ], !dbg !31
  call void @llvm.dbg.value(metadata i64 %for_index.0, metadata !24,
metadata !DIExpression()), !dbg !30
  switch i64 %for_index.0, label %Else [

Else:                                             ; preds = %ForCond
  %0 = add nuw i64 %for_index.0, 1, !dbg !31
  call void @llvm.dbg.value(metadata i64 %0, metadata !24, metadata
!DIExpression()), !dbg !30
  br label %ForCond, !dbg !31

ForEnd:                                           ; preds = %ForCond, %ForCond
  ret void, !dbg !33
}


I don't see how this follows from the original source. Is it a bug?