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In expr.c, this code
if (TREE_THIS_VOLATILE (exp))
MEM_VOLATILE_P (temp) = 1;
sets MEM_VOLATILE_P for a volatile MEM_REF. But this is not done for a volatile
TARGET_MEM_REF. This doesn't matter right now because tree-ssa-loop-ivopts.c
bails out if it sees a volatile memory reference, so volatile TARGET_MEM_REFs
don't (yet) occur in the wild.
It looks like set_mem_attributes () will set it via a similar check.