http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55442
Bug #: 55442
Summary: G++ uses up all my RAM when compiling a constexpr with
exponential call graph
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: da...@aitellu.com
I have compiled the following program on GCC 4.7.2 and the latest 4.8, using
Ubuntu 12.10 with Linux kernel 3.5.0:
const int MAXD = 24;
constexpr int count(int n, int depth=1){
return depth == MAXD ? n + 1: count(count(n, depth + 1), depth + 1) + 1;
}
#include<iostream>
int main(){
constexpr int i = count(0);
std::cout << i << std::endl;
}
Both versions of GCC will use over 3.3 gig RAM in about 30 seconds. For each
step I increase MAXD, the RAM usage will double until my computer swaps or the
kernel kills the process.
It will never reach a recursion depth of more than 24, but the call graph is
sort of a binary tree, so it will visit 2^MAXD - 1 nodes. Since the recursion
is so shallow, it should not have to use any memory. Clang 3.1 compiles it
without using "any" memory.
Before posting this report, I asked on Stackoverflow, where it was suggested I
report it here.
My guess is that this has something to do with unlimited memoization?
