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Vmgen recognizes the following strings in the C code part of simple instructions:
SET_IP
¶As far as Vmgen is concerned, a VM instruction containing this ends a VM basic block (used in profiling to delimit profiled sequences). On the C level, this also sets the instruction pointer.
SUPER_END
¶This ends a basic block (for profiling), even if the instruction contains no
SET_IP
.
INST_TAIL;
¶Vmgen replaces ‘INST_TAIL;’ with code for ending a VM instruction and dispatching the next VM instruction. Even without a ‘INST_TAIL;’ this happens automatically when control reaches the end of the C code. If you want to have this in the middle of the C code, you need to use ‘INST_TAIL;’. A typical example is a conditional VM branch:
if (branch_condition) { SET_IP(target); INST_TAIL; } /* implicit tail follows here */
In this example, ‘INST_TAIL;’ is not strictly necessary, because there is another one implicitly after the if-statement, but using it improves branch prediction accuracy slightly and allows other optimizations.
SUPER_CONTINUE
¶This indicates that the implicit tail at the end of the VM instruction
dispatches the sequentially next VM instruction even if there is a
SET_IP
in the VM instruction. This enables an optimization that is
not yet implemented in the vmgen-ex code (but in Gforth). The typical
application is in conditional VM branches:
if (branch_condition) { SET_IP(target); INST_TAIL; /* now this INST_TAIL is necessary */ } SUPER_CONTINUE;
Note that Vmgen is not smart about C-level tokenization, comments, strings, or conditional compilation, so it will interpret even a commented-out SUPER_END as ending a basic block (or, e.g., ‘RESET_IP;’ as ‘SET_IP;’). Conversely, Vmgen requires the literal presence of these strings; Vmgen will not see them if they are hiding in a C preprocessor macro.
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