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

[Alberto Barbaro via llvm-dev]

Hi David,
My question is: why both #ifdef and #else branches are missing? I think at
least one of the two should be present... In fact there is a case where the
width could be greater then
PNG_USER_WIDTH_MAX but not greater then PNG_UINT_31_MAX. That's why I was
expecting at least one of the two...

Thanks
Alberto



On Wed, Dec 18, 2019, 22:12 David Blaikie <dblaikie at gmail.com> wrote:

> 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
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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