Compiled constant expr code must still emit all op execute hooks

[mvorisek]

Description

related with sebastianbergmann/php-code-coverage#889

if code can be run on compile time (constant expr), all hooks, used for ex. by coverage libs like xdebug, must still be called in the same order as if the code was not optimized on compile time

examples:

code with always constant expr:

        $v2
            =
            (bool)
            1
        ;

the cast operation can be always evaluated/optimized on compile time, but cast operation hook must be always emit

code with sometimes constant expr:

        $v3
            =
            (bool)
            AType::OPT
        ;

same as the first example when AType class was loaded before this example was included/compiled

[bwoebi]

For debugging and tracing purposes, PHP modules can selectively disable some optimizations, e.g. ZEND_COMPILE_NO_(PERSISTENT_)CONSTANT_SUBSTITUTION, ZEND_COMPILE_NO_BUILTINS.

For the purposes of pure constant expressions like (bool)1, this can be discovered by a code-coverage tool by looking at the AST (e.g. nikic/php-parser), to infer that the lines (bool) and 1 are constant and always together.

As such, there is no explicit need to change this, and changing this would have a measurable performance impact, so, we won't do that.

[mvorisek]

For debugging and tracing purposes, PHP modules can selectively disable some optimizations, e.g. ZEND_COMPILE_NO_(PERSISTENT_)CONSTANT_SUBSTITUTION, ZEND_COMPILE_NO_BUILTINS.

when a module will disable the optimizations, will an already compiled/cached scripts be recompiled?

[bwoebi]

Yes, there exists an API zend_add_system_entropy for that use case.