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[docs][PerformanceTips] Add text on allocas and alignment

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This summarizes two recent llvm-dev discussions.  Most of the text provided by David Chisnall and Benoit Belley with minor editting by me.  



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247301 91177308-0d34-0410-b5e6-96231b3b80d8
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Philip Reames 2015-09-10 17:03:10 +00:00
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docs/Frontend/PerformanceTips.rst
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@ -46,6 +46,22 @@ The Basics
perform badly with confronted with such structures. The only exception to
this guidance is that a unified return block with high in-degree is fine.
Use of allocas
^^^^^^^^^^^^^^
An alloca instruction can be used to represent a function scoped stack slot,
but can also represent dynamic frame expansion. When representing function
scoped variables or locations, placing alloca instructions at the beginning of
the entry block should be preferred. In particular, place them before any
call instructions. Call instructions might get inlined and replaced with
multiple basic blocks. The end result is that a following alloca instruction
would no longer be in the entry basic block afterward.
The SROA (Scalar Replacement Of Aggregates) and Mem2Reg passes only attempt
to eliminate alloca instructions that are in the entry basic block. Given
SSA is the canonical form expected by much of the optimizer; if allocas can
not be eliminated by Mem2Reg or SROA, the optimizer is likely to be less
effective than it could be.
Avoid loads and stores of large aggregate type
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@ -79,6 +95,31 @@ operations for safety. If your source language provides information about
the range of the index, you may wish to manually extend indices to machine
register width using a zext instruction.
When to specify alignment
^^^^^^^^^^^^^^^^^^^^^^^^^^
LLVM will always generate correct code if you don’t specify alignment, but may
generate inefficient code. For example, if you are targeting MIPS (or older
ARM ISAs) then the hardware does not handle unaligned loads and stores, and
so you will enter a trap-and-emulate path if you do a load or store with
lower-than-natural alignment. To avoid this, LLVM will emit a slower
sequence of loads, shifts and masks (or load-right + load-left on MIPS) for
all cases where the load / store does not have a sufficiently high alignment
in the IR.
The alignment is used to guarantee the alignment on allocas and globals,
though in most cases this is unnecessary (most targets have a sufficiently
high default alignment that they’ll be fine). It is also used to provide a
contract to the back end saying ‘either this load/store has this alignment, or
it is undefined behavior’. This means that the back end is free to emit
instructions that rely on that alignment (and mid-level optimizers are free to
perform transforms that require that alignment). For x86, it doesn’t make
much difference, as almost all instructions are alignment-independent. For
MIPS, it can make a big difference.
Note that if your loads and stores are atomic, the backend will be unable to
lower an under aligned access into a sequence of natively aligned accesses.
As a result, alignment is mandatory for atomic loads and stores.
Other Things to Consider
^^^^^^^^^^^^^^^^^^^^^^^^