[llvm-dev] Missing code depending on a #ifdef within the .ll file

[David Blaikie via llvm-dev]

Not sure I understand the question - but usually a compiler would, on one
compilation, only compile one versoin of the code with one set of defines.

There could be invalid code (it could literally be junk - or it could be
code that's only valid when the define is true/false, etc) in an
unreachable ifdef, so it doesn't make sense to compile it.

On Wed, Dec 18, 2019 at 2:09 PM Alberto Barbaro via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
> I have managed to compile libpng using wllvm and obtain the IR of pngpixel
> ( small tool which is part of libpng ).
>
> libpng has a function called png_check_IHDR:
>
> void /* PRIVATE */
> png_check_IHDR(png_const_structrp png_ptr,
>    png_uint_32 width, png_uint_32 height, int bit_depth,
>    int color_type, int interlace_type, int compression_type,
>    int filter_type)
> {
>    int error = 0;
>
>    /* Check for width and height valid values */
>    if (width == 0)
>    {
>       png_warning(png_ptr, "Image width is zero in IHDR");
>       error = 1;
>    }
>    else if (width > PNG_UINT_31_MAX)
>    {
>       png_warning(png_ptr, "Invalid image width in IHDR");
>       error = 1;
>    }
>
>
>
>
>
>
>
>
>
>
>
> * else   {#     ifdef PNG_SET_USER_LIMITS_SUPPORTED      if (width >
> png_ptr->user_width_max)#     else      if (width > PNG_USER_WIDTH_MAX)#
>   endif      {         png_warning(png_ptr, "Image width exceeds user limit
> in IHDR");         error = 1;      }   }*
> ...
> }
>
> Unfortunately the relative IR does not seem to have the branch highlighted
> in bold:
>
> ; Function Attrs: nounwind uwtable
> define void @png_check_IHDR(%struct.png_struct_def* noalias %png_ptr, i32
> %width, i32 %height, i32 %bit_depth, i32 %color_type, i32 %interlace_type,
> i32 %compression_type, i32 %filter_type) #5 {
>   *%1 = icmp eq i32 %width, 0*
>   br i1 %1, label %.thread, label %2
>
> .thread:                                          ; preds = %0
>   tail call void @png_warning(%struct.png_struct_def* %png_ptr, i8*
> getelementptr inbounds ([28 x i8], [28 x i8]* @.str50, i64 0, i64 0)) #15
>   br label %5
>
> ; <label>:2:                                      ; preds = %0
>   *%3 = icmp slt i32 %width, 0*
>   br i1 %3, label %4, label %5
>
> ; <label>:4:                                      ; preds = %2
>   tail call void @png_warning(%struct.png_struct_def* %png_ptr, i8*
> getelementptr inbounds ([28 x i8], [28 x i8]* @.str51, i64 0, i64 0)) #15
>   br label %5
>
> ; <label>:5:                                      ; preds = %4, %2,
> %.thread
>   %error.1 = phi i32 [ 1, %4 ], [ 0, %2 ], [ 1, %.thread ]
>   %6 = getelementptr inbounds %struct.png_struct_def,
> %struct.png_struct_def* %png_ptr, i64 0, i32 132
>   %7 = load i32, i32* %6, align 4, !tbaa !22
>   *%8 = icmp ult i32 %7, %width ; if (width > PNG_UINT_31_MAX)*
>   br i1 %8, label %9, label %10
>
> ; <label>:9:                                      ; preds = %5
>   tail call void @png_warning(%struct.png_struct_def* %png_ptr, i8*
> getelementptr inbounds ([39 x i8], [39 x i8]* @.str53, i64 0, i64 0)) #15
>   br label %10
>
> ; <label>:10:                                     ; preds = %9, %5
>   %error.3 = phi i32 [ 1, %9 ], [ %error.1, %5 ]
>   *%11 = icmp eq i32 %height, 0 ; first check on the height*
>   br i1 %11, label %.thread27, label %12
>
> Does anybody know why the else branch is completely missing?
>
> Thanks,
> Alberto
>
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