GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction

[Jakub Jelinek]

On Fri, Apr 13, 2018 at 01:34:21PM +0000, Vivek Kinhekar wrote:
> Hello Alexander,
> 
> In the given testcase, the generated fdivrs instruction performs the
> division of a symbol ref (memory value) by FPU Stack Register and stores
> the value in FPU Stack Register.

The stack registers are not memory.

> Please find the following RTL Dump of the fdivrs instruction generated. 
> It clearly access the memory for read access! 

That is a constant read, that doesn't count either.  It is in memory only
because the instruction doesn't support constant immediates, the memory is
read-only.

	Jakub