487200 - Do not embed stack address constants in generated code

Status: VERIFIED FIXED

(Tamarin Graveyard :: Virtual Machine, defect)

Description

[Edwin Smith]

Doing so ties the code to a specific thread (well, stack).  Instead load the minstack value from AvmCore (which can be const).

Comment 1

[Edwin Smith]

The only change I made was to change this:

LIns *c = binaryIns(LIR_pult, vars, InsConstPtr((void*)core->minstack));
LIns *b = branchIns(LIR_jf, c);

to this:

LIns *d = loadIns(LIR_ldp, 0, InsConstPtr(&core->minstack));
LIns *c = binaryIns(LIR_pult, vars, d);
LIns *b = branchIns(LIR_jf, c);

--tinic

Comment 2

[Lars T Hansen]

redux changeset:   1777:ff76cf536103

Comment 3

[Edwin Smith]

this change enables an optimization to stack overflow checks/handling

before this change we compare sp with a constant, and call a handler if we have overflowed.  Handling a stack overflow involves calling yet more functions that deal with the error, and those functions should not fail.  So, core->stackOverflow() is stateful, and ignores additional errors (since we're now in the stack slop/reserve area).

With this patch, we compare sp with a loaded value.  The value can be changed after an initial error, to avoid needing this statefulness in the overflow handler.

furthermore the call to the handler (env->stkover()) must currently be a *call*, because of the statefulness.  (sometimes the call returns and the code must continue).  we could optimize generated code so that the call instead is a jump that can never return.  (or a call known not to return).