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

Wed Dec 18 15:14:31 PST 2019

Hi Paul and David,
yes I have just realised that the third check was the one I was looking
for... For some reason I was only looking for *PNG_USER_WIDTH_MAX.*

Now it is all clear :)

Thanks again
Alberto

Il giorno mer 18 dic 2019 alle ore 23:03 Robinson, Paul <
paul.robinson at sony.com> ha scritto:

>                if (width == 0)
>
> became
>
>                %1 = icmp eq i32 %width, 0
>
> which is straightforward; and
>
>                else if (width > PNG_UINT_31_MAX)
>
> became (assuming the constant is 0x7fffffff)
>
>                %3 icmp slt i32 %width, 0
>
> (note this is a signed comparison with 0, which is equivalent to the
> comparison in the source)
>
> and finally
>
>                if (width > png_ptr->user_width_max)
>
> became
>
>                %8 = icmp ult i32 %7, %width
>
> after loading the png_ptr field into %7 in the previous instruction.
>
>
>
> Does that help?
>
> --paulr
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *David
> Blaikie via llvm-dev
> *Sent:* Wednesday, December 18, 2019 5:30 PM
> *To:* Alberto Barbaro <barbaro.alberto at gmail.com>
> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] Missing code depending on a #ifdef within the
> .ll file
>
>
>
> I'm sorry, I don't understand the question - the source code shows 3 calls
> to png_warning (one of them from the if block you're interested in), and
> the IR shows 3 calls to png_warning - so it looks like the body of the if
> block is included in the IR you've shown?
>
>
>
> On Wed, Dec 18, 2019 at 2:24 PM Alberto Barbaro <barbaro.alberto at gmail.com>
> wrote:
>
> 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
>
>
>
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