Re: Widening multiplication, but no narrowing division [i386/AMD64]

[LIU Hao via Gcc]

在 2023/1/9 20:20, Stefan Kanthak 写道:

Hi,

GCC (and other C compilers too) support the widening multiplication
of i386/AMD64 processors, but DON'T support their narrowing division:

QWORD-DWORD division would change the behavior of your program.


Given:

   ```
   uint32_t xdiv(uint64_t x, uint32_t y) { return x / y;  }
   ```




If DIV was used, it would effect an exception:

   ```
   mov edx, 2
   mov eax, edx   # edx:eax = 0x200000002

   mov ecx, edx
   div ecx        # division overflows because the quotient
                  # can't stored into EAX
   ```





--
Best regards,
LIU Hao

then `xdiv(0x200000002, 2)` should first convert both operands to `uint64_t`, perform the division which yields `0x100000001`, then truncate the quotient to 32-bit which gives `1`. The result is exact.

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