Use returns_nonnull in BumpPtrAllocator and MallocAllocator to avoid null-check in placement new

In both Clang and LLVM, this is a common pattern:

  Size = sizeof(DeclRefExpr) + SomeExtraStuff;
  void *Mem = Context.Allocate(Size, llvm::alignOf<DeclRefExpr>());
  return new (Mem) DeclRefExpr(...);

The annoying thing is that because the default placement-new operator has a
nothrow specification, the compiler will insert a null check of Mem before
calling the DeclRefExpr constructor. This null check is redundant for us,
because we expect the allocation functions to never return null.

By annotating the allocator functions with returns_nonnull, we can optimize
away these checks. Compiling clang with a recent version of Clang and measuring
with:

  $ perf stat -r20 bin/clang.patch -fsyntax-only -w gcc.c && perf stat -r20 bin/clang.orig -fsyntax-only -w gcc.c

Shows a 2.4% speed-up (+- 0.8%).

The pattern occurs in LLVM too. Measuring with -O3 (and now using bzip2.c
instead, because it's smaller):

  $ perf stat -r20 bin/clang.patch -O3 -w bzip2.c  &&  perf stat -r20 bin/clang.orig -O3 -w bzip2.c

Shows 4.4 % speed-up (+- 1%).

If anyone knows of a similar attribute we can use for MSVC, or some other
technique to get rid off the null check there, please let me know.

Differential Revision: http://reviews.llvm.org/D4989

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216192 91177308-0d34-0410-b5e6-96231b3b80d8
Thumb1 load/store optimizer: Improve code to materialize new base register.

There are two add-immediate instructions in Thumb1: tADDi8 and tADDi3. Only
the latter supports using different source and destination registers, so
whenever we materialize a new base register (at a certain offset) we'd do
so by moving the base register value to the new register and then adding in
place. This patch changes the code to use a single tADDi3 if the offset is
small enough to fit in 3 bits.

Differential Revision: http://reviews.llvm.org/D5006

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216193 91177308-0d34-0410-b5e6-96231b3b80d8