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A new LLDS->LLDS transformation that optimizes instruction sequences such
as the following extract from tree234__search:
MR_r1 = MR_stackvar(3);
MR_r2 = MR_stackvar(4);
MR_r3 = MR_const_field(MR_mktag(1), MR_stackvar(1), (MR_Integer) 2);
MR_r4 = MR_stackvar(2);
MR_succip = (MR_Code *) MR_stackvar(5);
if ((MR_tag(MR_r3) != MR_mktag((MR_Integer) 1))) {
MR_GOTO_LABEL(mercury__x3__search_3_0_i1);
}
MR_stackvar(1) = MR_r3;
MR_stackvar(2) = MR_r4;
MR_stackvar(3) = MR_r1;
MR_stackvar(4) = MR_r2;
MR_r2 = MR_r4;
MR_r3 = MR_const_field(MR_mktag(1), MR_r3, (MR_Integer) 0);
MR_call_localret(...)
The code before the if-then-else is part of the procedure epilogue; the code
after it is the code from the initial part of the procedure that fulljump
optimization replaces the self-tail-call with.
The transformation deletes the redundant assignments to stackvars 2, 3 and 4.
It reduces both the size and the runtime of the compiler by about 0.5%.
compiler/reassign.m:
The new module that does the work.
compiler/optimize.m:
Invoke the new module if the optimization is enabled. Invoke it after
most other optimizations have been run, since they may create more
opportunities for it.
compiler/option.m:
Add a new option to control whether the new optimization is enabled.
Turn on the new optimization at optimization level 3.
doc/user_guide.texi:
Document the new option.
compiler/notes/compiler_design.html:
Document the new module.