From: Eric Dumazet

Sent: 01 March 2017 17:33
On Wed, 2017-03-01 at 08:14 -0800, Alexander Duyck wrote:

quoted

What build flags are you using?  With -Os or -O2 I have seen it
convert the /b * c into a single shift.

Because b & c are unsigned in our case.

I presume David tried signed integers, this is why gcc does that.

I was using integer constants but an unsigned variable.
Changing the constants to unsigned makes no difference (they would get
promoted anyway).

After some experiments I can get gcc to generate a single shift if the
variable being tested is 32bits, the constants are 64bits and a 64bit
result is required.
In every other case it does either and-shr-shl or shr-and-shl (all the
shr are logical).

	David