Add a few more performance tips

These came from my own experience and may not apply equally to all use cases.  Any alternate perspective anyone has should be used to refine these.  

As always, grammar and spelling adjustments are more than welcome.  Please just directly commit a fix if you see something problematic.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231352 91177308-0d34-0410-b5e6-96231b3b80d8

Author: preames

docs/Frontend/PerformanceTips.rst

@@ -53,13 +53,25 @@ Other things to consider
 #. Make sure that a DataLayout is provided (this will likely become required in
    the near future, but is certainly important for optimization).
 
-#. Add nsw/nuw/fast-math flags as appropriate
+#. Add nsw/nuw flags as appropriate.  Reasoning about overflow is 
+   generally hard for an optimizer so providing these facts from the frontend 
+   can be very impactful.  For languages which need overflow semantics, 
+   consider using the :ref:`overflow intrinsics <int_overflow>`.
+
+#. Use fast-math flags on floating point operations if legal.  If you don't 
+   need strict IEEE floating point semantics, there are a number of additional 
+   optimizations that can be performed.  This can be highly impactful for 
+   floating point intensive computations.
+
+#. Use inbounds on geps.  This can help to disambiguate some aliasing queries.
 
 #. Add noalias/align/dereferenceable/nonnull to function arguments and return 
    values as appropriate
 
-#. Mark functions as readnone/readonly/nounwind when known (especially for 
-   external functions)
+#. Mark functions as readnone/readonly or noreturn/nounwind when known.  The 
+   optimizer will try to infer these flags, but may not always be able to.  
+   Manual annotations are particularly important for external functions that 
+   the optimizer can not analyze.
 
 #. Use ptrtoint/inttoptr sparingly (they interfere with pointer aliasing 
    analysis), prefer GEPs
@@ -85,9 +97,51 @@ Other things to consider
    and may not be well optimized by the current optimizer.  Depending on your
    source language, you may consider using fences instead.
 
+#. If calling a function which is known to throw an exception (unwind), use 
+   an invoke with a normal destination which contains an unreachable 
+   instruction.  This form conveys to the optimizer that the call returns 
+   abnormally.  For an invoke which neither returns normally or requires unwind
+   code in the current function, you can use a noreturn call instruction if 
+   desired.  This is generally not required because the optimizer will convert
+   an invoke with an unreachable unwind destination to a call instruction.
+
 #. If you language uses range checks, consider using the IRCE pass.  It is not 
    currently part of the standard pass order.
 
+#. For languages with numerous rarely executed guard conditions (e.g. null 
+   checks, type checks, range checks) consider adding an extra execution or 
+   two of LoopUnswith and LICM to your pass order.  The standard pass order, 
+   which is tuned for C and C++ applications, may not be sufficient to remove 
+   all dischargeable checks from loops.
+
+#. Use profile metadata to indicate statically known cold paths, even if 
+   dynamic profiling information is not available.  This can make a large 
+   difference in code placement and thus the performance of tight loops.
+
+#. When generating code for loops, try to avoid terminating the header block of
+   the loop earlier than necessary.  If the terminator of the loop header 
+   block is a loop exiting conditional branch, the effectiveness of LICM will
+   be limited for loads not in the header.  (This is due to the fact that LLVM 
+   may not know such a load is safe to speculatively execute and thus can't 
+   lift an otherwise loop invariant load unless it can prove the exiting 
+   condition is not taken.)  It can be profitable, in some cases, to emit such 
+   instructions into the header even if they are not used along a rarely 
+   executed path that exits the loop.  This guidance specifically does not 
+   apply if the condition which terminates the loop header is itself invariant,
+   or can be easily discharged by inspecting the loop index variables.
+
+#. In hot loops, consider duplicating instructions from small basic blocks 
+   which end in highly predictable terminators into their successor blocks.  
+   If a hot successor block contains instructions which can be vectorized 
+   with the duplicated ones, this can provide a noticeable throughput
+   improvement.  Note that this is not always profitable and does involve a 
+   potentially large increase in code size.
+
+#. Avoid high in-degree basic blocks (e.g. basic blocks with dozens or hundreds
+   of predecessors).  Among other issues, the register allocator is known to 
+   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.
+
 p.s. If you want to help improve this document, patches expanding any of the 
 above items into standalone sections of their own with a more complete 
 discussion would be very welcome.  

docs/LangRef.rst

@@ -9055,6 +9055,8 @@ then the result is the size in bits of the type of ``src`` if
 ``is_zero_undef == 0`` and ``undef`` otherwise. For example,
 ``llvm.cttz(2) = 1``.
 
+.. _int_overflow:
+
 Arithmetic with Overflow Intrinsics
 -----------------------------------