[PATCH] D117356: InstructionCombining: avoid eliding mismatched alloc/free pairs

[Augie Fackler via Phabricator via llvm-commits]

durin42 created this revision.
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Prior to this change LLVM would happily elide a call to any allocation
function and a call to any free function operating on the same unused
pointer. This can cause problems in some obscure cases, for example if
the body of operator::new can be inlined but the body of
operator::delete can't, as in this example from jyknight:

  #include <stdlib.h>
  #include <stdio.h>
  
  int allocs = 0;
  
  void *operator new(size_t n) {
      allocs++;
      void *mem = malloc(n);
      if (!mem) abort();
      return mem;
  }
  
  __attribute__((noinline)) void operator delete(void *mem) noexcept {
      allocs--;
      free(mem);
  }
  
  void deleteit(int*i) { delete i; }
  int main() {
      int*i = new int;
      deleteit(i);
      if (allocs != 0)
        printf("MEMORY LEAK! allocs: %d\n", allocs);
  }

This patch addresses the issue by introducing the concept of an
allocator function family and uses it to make sure that alloc/free
function pairs are only removed if they're in the same family.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D117356

Files:
  llvm/include/llvm/Analysis/MemoryBuiltins.h
  llvm/lib/Analysis/MemoryBuiltins.cpp
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/malloc-free-mismatched.ll

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