[Lldb-commits] [PATCH] Don't use a random probe / allocation scheme in the IRMemoryMap

[Zachary Turner]

+scallanan

I tried to add you to the review through Phab, but it looks like you
weren't registered.  Thanks


On Wed, Jun 25, 2014 at 4:15 PM, Zachary Turner <zturner at google.com> wrote:

> The current strategy for host allocation is to choose a random address and
> attempt to allocate there, eventually failing if the allocation cannot be
> satisfied.
>
> The C standard only guarantees that RAND_MAX >= 32767, so for platforms
> where this is true allocations will fail with very high probability.   On
> such platforms, you can reproduce this trivially by running lldb, typing
> "expr (3)" and then hitting enter you see a failure.  Failures generally
> happen with a frequency of about 1 failure every 5 evaluations.
>
> I cannot come up with a good reason that the allocations need to look like
> "real" pointers, so this patch changes the allocation scheme to simply jump
> straight to the end and grab a free chunk of memory.
>
> http://reviews.llvm.org/D4300
>
> Files:
>   source/Expression/IRMemoryMap.cpp
>
> Index: source/Expression/IRMemoryMap.cpp
> ===================================================================
> --- source/Expression/IRMemoryMap.cpp
> +++ source/Expression/IRMemoryMap.cpp
> @@ -53,6 +53,8 @@
>      lldb::ProcessSP process_sp = m_process_wp.lock();
>
>      lldb::addr_t ret = LLDB_INVALID_ADDRESS;
> +    if (size == 0)
> +      return ret;
>
>      if (process_sp && process_sp->CanJIT() && process_sp->IsAlive())
>      {
> @@ -66,37 +68,13 @@
>              return ret;
>      }
>
> -    for (int iterations = 0; iterations < 16; ++iterations)
> -    {
> -        lldb::addr_t candidate = LLDB_INVALID_ADDRESS;
> -
> -        switch (target_sp->GetArchitecture().GetAddressByteSize())
> -        {
> -        case 4:
> -            {
> -                uint32_t random_data = rand();
> -                candidate = random_data;
> -                candidate &= ~0xfffull;
> -                break;
> -            }
> -        case 8:
> -            {
> -                uint32_t random_low = rand();
> -                uint32_t random_high = rand();
> -                candidate = random_high;
> -                candidate <<= 32ull;
> -                candidate |= random_low;
> -                candidate &= ~0xfffull;
> -                break;
> -            }
> -        }
> -
> -        if (IntersectsAllocation(candidate, size))
> -            continue;
> -
> -        ret = candidate;
> -
> -        return ret;
> +    // Don't allocate from the zero page.
> +    ret = 0x1000;
> +    if (!m_allocations.empty()) {
> +      auto back = m_allocations.rbegin();
> +      lldb::addr_t addr = back->first;
> +      size_t size = back->second.m_size;
> +      ret = llvm::RoundUpToAlignment(addr+size, 4096);
>      }
>
>      return ret;
>
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