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

[Andrew Kelley via llvm-dev]

2 things

1. I misread the LLVM IR. It's not an infinite loop, it's just a
missed optimization.

2. I can reproduce this with a dead simple C program:

void entry(void) {
    for (unsigned i = 0; i != 10; i += 1) {
        if (i == 1) break;
    }
}
https://godbolt.org/z/m3sjkA

On Fri, Nov 23, 2018 at 3:47 PM Andrew Kelley <superjoe30 at gmail.com> wrote:
>
> Here's a godbolt link: https://godbolt.org/z/BaWZrM
> And here's the expected @entry function after optimization:
>
> define void @entry() local_unnamed_addr #0 {
> Entry:
>   ret void
> }
>
>
>
> On Fri, Nov 23, 2018 at 3:32 PM Andrew Kelley <superjoe30 at gmail.com> wrote:
> >
> > 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?