What, in short words, does the GCC option -fipa-pta do?

Question

According to the GCC manual, the -fipa-pta optimization does:

-fipa-pta: Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level.

What I assume is that GCC tries to differentiate mutable and immutable data based on pointers and references used in a procedure. Can someone with more in-depth GCC knowledge explain what -fipa-pta does?

Answer 1

I think the word "interprocedural" is the key here.

I'm not intimately familiar with gcc's optimizer, but I've worked on optimizing compilers before. The following is somewhat speculative; take it with a small grain of salt, or confirm it with someone who knows gcc's internals.

An optimizing compiler typically performs analysis and optimization only within each individual function (or subroutine, or procedure, depending on the language). For example, given code like this contrived example:

double *ptr = ...;

void foo(void) {
    ...
    *ptr = 123.456;
    some_other_function();
    printf("*ptr = %f\n", *ptr);
}

the optimizer will not be able to determine whether the value of *ptr has been changed by the call to some_other_function().

If interprocedural analysis is enabled, then the optimizer can analyze the behavior of some_other_function(), and it may be able to prove that it can't modify *ptr. Given such analysis, it can determine that the expression *ptr must still evaluate to 123.456, and in principle it could even replace the printf call with puts("ptr = 123.456");.

(In fact, with a small program similar to the above code snippet I got the same generated code with -O3 and -O3 -fipa-pta, so I'm probably missing something.)

Since a typical program contains a large number of functions, with a huge number of possible call sequences, this kind of analysis can be very expensive.