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10.2.4.5 Selection Statements
A simple selection statement, such as the C if statement, is
expressed in GIMPLE using a void COND_EXPR. If only one branch is
used, the other is filled with an empty statement.
Normally, the condition expression is reduced to a simple comparison. If
it is a shortcut (&& or ||) expression, however, we try to
break up the if into multiple ifs so that the implied shortcut
is taken directly, much like the transformation done by do_jump in
the RTL expander.
A SWITCH_EXPR in GIMPLE contains the condition and a
TREE_VEC of CASE_LABEL_EXPRs describing the case values
and corresponding LABEL_DECLs to jump to. The body of the
switch is moved after the SWITCH_EXPR.