[cfe-dev] LLVM 3.6: Bad IR generation

Tue Mar 17 10:53:46 PDT 2015

On Tue, Mar 17, 2015 at 10:45 AM, James Brock <
james at cognitive-electronics.com> wrote:

> I’m getting some weird results in the text of my LLVM-IR when compiling
> using version 3.6 with the -g option. For a simple c function (in a file
> func.c, shown below)
>
> void test_func(unsigned int * dIn, unsigned int * dOut) {
>    unsigned int idx = 0;
>    for (idx = 0; idx < 100; idx++) {
>       dOut[idx] = dIn[idx];
>    }
>    return;
> }
>
> and the compile command shown below
>
> /path/to/clang -target x86_64 -v -g -S -emit-llvm -o func.ll func.c
>
> The LLVM IR in the resultant .ll file (in part included below) contains
> strings with a lot of unusual and unnecessary characters, including a lot
> of null characters ‘\000’. This seams to be in the part of the LLVM IR
> related to the Dwarf information, and goes away with the removal of the -g
> option from the command line. I’ve tried this with a number of different
> optimization levels and target architectures with the same results. Any
> help or explanation for the odd strings is greatly appreciated.
>

That's by design:

http://llvm.org/docs/SourceLevelDebugging.html#id10

"Most of the string and integer fields in descriptors are packed into a
single, null-separated mdstring. "

>
>
> ; Function Attrs: nounwind readnone
> declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
>
> attributes #0 = { nounwind uwtable "less-precise-fpmad"="false"
> "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
> "no-infs-fp-math"="false" "no-nans-fp-math"="false"
> "stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
> "use-soft-float"="false" }
> attributes #1 = { nounwind readnone }
>
> !llvm.dbg.cu = !{!0}
> !llvm.module.flags = !{!10, !11}
> !llvm.ident = !{!12}
>
> !0 = !{!"0x11\0012\00clang version 3.6.0
> (tags/RELEASE_360/final)\000\00\000\00\001", !1, !2, !2, !3, !2, !2} ; [
> DW_TAG_compile_unit ]
> [/home/james/workspace/Libraries/opencl/coge_ocl/trunk/test/dat/ocl_kern1.c]
> [DW_LANG_C99]
> !1 = !{!"ocl_kern1.c",
> !"/home/james/workspace/Libraries/opencl/coge_ocl/trunk/test/dat"}
> !2 = !{}
> !3 = !{!4}
> !4 =
> !{!"0x2e\00test_func\00test_func\00\002\000\001\000\000\00256\000\002", !1,
> !5, !6, null, void (i32*, i32*)* @test_func, null, null, !2} ; [
> DW_TAG_subprogram ] [line 2] [def] [test_func]
> !5 = !{!"0x29", !1}                               ; [ DW_TAG_file_type ]
> [/home/james/workspace/Libraries/opencl/coge_ocl/trunk/test/dat/ocl_kern1.c]
> !6 = !{!"0x15\00\000\000\000\000\000\000", null, null, null, !7, null,
> null, null} ; [ DW_TAG_subroutine_type ] [line 0, size 0, align 0, offset
> 0] [from ]
> !7 = !{null, !8, !8}
> !8 = !{!"0xf\00\000\0064\0064\000\000", null, null, !9} ; [
> DW_TAG_pointer_type ] [line 0, size 64, align 64, offset 0] [from unsigned
> int]
> !9 = !{!"0x24\00unsigned int\000\0032\0032\000\000\007", null, null} ; [
> DW_TAG_base_type ] [unsigned int] [line 0, size 32, align 32, offset 0, enc
> DW_ATE_unsigned]
> !10 = !{i32 2, !"Dwarf Version", i32 4}
> !11 = !{i32 2, !"Debug Info Version", i32 2}
> !12 = !{!"clang version 3.6.0 (tags/RELEASE_360/final)"}
> !13 = !{!"0x101\00dIn\0016777218\000", !4, !5, !8} ; [ DW_TAG_arg_variable
> ] [dIn] [line 2]
> !14 = !{!"0x102"}                                 ; [ DW_TAG_expression ]
> !15 = !MDLocation(line: 2, column: 31, scope: !4)
> !16 = !{!"0x101\00dOut\0033554434\000", !4, !5, !8} ; [
> DW_TAG_arg_variable ] [dOut] [line 2]
> !17 = !MDLocation(line: 2, column: 51, scope: !4)
> !18 = !{!"0x100\00idx\003\000", !4, !5, !9}       ; [ DW_TAG_auto_variable
> ] [idx] [line 3]
> !19 = !MDLocation(line: 3, column: 17, scope: !4)
> !20 = !MDLocation(line: 5, column: 9, scope: !21)
> !21 = !{!"0xb\005\004\000", !1, !4}               ; [ DW_TAG_lexical_block
> ]
> [/home/james/workspace/Libraries/opencl/coge_ocl/trunk/test/dat/ocl_kern1.c]
> !22 = !MDLocation(line: 5, column: 18, scope: !23)
> !23 = !{!"0xb\002", !1, !24}                      ; [ DW_TAG_lexical_block
> ]
> [/home/james/workspace/Libraries/opencl/coge_ocl/trunk/test/dat/ocl_kern1.c]
> !24 = !{!"0xb\001", !1, !25}                      ; [ DW_TAG_lexical_block
> ]
> [/home/james/workspace/Libraries/opencl/coge_ocl/trunk/test/dat/ocl_kern1.c]
> !25 = !{!"0xb\005\004\001", !1, !21}              ; [ DW_TAG_lexical_block
> ]
> [/home/james/workspace/Libraries/opencl/coge_ocl/trunk/test/dat/ocl_kern1.c]
> !26 = !MDLocation(line: 5, column: 4, scope: !21)
> !27 = !MDLocation(line: 6, column: 23, scope: !28)
> !28 = !{!"0xb\005\0036\002", !1, !25}             ; [ DW_TAG_lexical_block
> ]
> [/home/james/workspace/Libraries/opencl/coge_ocl/trunk/test/dat/ocl_kern1.c]
> !29 = !MDLocation(line: 6, column: 19, scope: !28)
> !30 = !MDLocation(line: 6, column: 12, scope: !28)
> !31 = !MDLocation(line: 6, column: 7, scope: !28)
> !32 = !MDLocation(line: 7, column: 4, scope: !28)
> !33 = !MDLocation(line: 5, column: 29, scope: !25)
> !34 = !MDLocation(line: 5, column: 4, scope: !25)
> !35 = !MDLocation(line: 9, column: 4, scope: !4)
>
>
>
> James Brock
>
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> cfe-dev at cs.uiuc.edu
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>
>
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