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Re: Make THISFN simpler and more powerful [message #60927 is a reply to message #60926]

Wed, 09 October 2024 16:09

Lance
Messages: 656
Registered: March 2007

Contributor

I don't like that fact that compilers are allowed to stuff random padding bits in a bitfield as they like, but it's actually standard compliant.

In the above example, change unsigned to byte (Upp::byte of course) actually removes the extra cost on total storage usage. But the padding MSVC inserts vs GCC's sequential packed bits will result in binary incompatibilities. Worse, some old c tricks no longer work with MSVC.

A somewhat more realistic though simplified example.

class SomeFormat{
...
private:
    Font font;
    Color paper, ink, highlight;
    union{
       int32 dummy;
       struct{
           byte info1:3;
           byte info2:5;
           
           // allow individual font properties
           // to be Null for multi-tier composition
           bool faceNotNull:1;
           bool heightNotNull:1;
           bool widthNotNull:1;
           bool boldNotNull:1;
           bool strikeoutNotNull:1;
           bool underlineNotNull:1;
           bool italicNotNull:1;
           
       };
    };

    
};

In old c days, if we want to check if all Font properties are set, we can simply

bool SomeFormat::AllFontPropertiesSet()const
{
#define SOMEFORMAT_MASK (((1<<7)-1)<<8)
    return (dummy & SOMEFORMAT_MASK) == SOMEFORMAT_MASK;
#define SOMEFORMAT_MASK
}

And to mark all font properties as set(non-Null)

SomeFormat::SetAllFontProperties()
{
#define SOMEFORMAT_MASK (((1<<7)-1)<<8)
    return dummy |= SOMEFORMAT_MASK;
#define SOMEFORMAT_MASK
}

etc. With GCC, you can still do things like that. Total predictability. Fully appreciated.

End of the day, what benefits MSVC is going to achieve by padding random bits? I can see if a bitfield crosses a machine's fast-integer boundary (a few bits in previous FAST-INTEGER and a few in the following), there will be extra cpu cost involved. Other than that, what's going to be saved?

Thumbs down for MSVC on this regard.

[Updated on: Wed, 09 October 2024 16:12]

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