[LLVMdev] llvm-gcc miscompilation or it's the gcc's rule?

[Zhou Sheng]

Hi,

Here is C function:

uint64_t mul(uint32_t x, uint32_t y) {
  return x * y;
}

current llvm-gcc-4.0 with -O3 will compile it to:

define i64 @mul(i32 %x, i32 %y) nounwind  {
entry:
    %tmp3 = mul i32 %y, %x      ; <i32> [#uses=1]
    %tmp34 = zext i32 %tmp3 to i64      ; <i64> [#uses=1]
    ret i64 %tmp34
}

This seems incorrect. I think it should extend %x, %y to i64 first and then
do the multiplication stuff.
Otherwise, the result may lose some bits if %x, %y are very big.

gcc seems have the same issue. Is this a bug or just gcc's rule?


Thanks,
Sheng.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20080113/d59fab67/attachment.html>