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

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

docs/Frontend/PerformanceTips.rst

@@ -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
 ^^^^^^^^^^^^^^^^^^^^^^^^