Add new llvm.loop.unroll.enable metadata for use with "#pragma unroll".

This change adds the new unroll metadata "llvm.loop.unroll.enable" which directs
the optimizer to unroll a loop fully if the trip count is known at compile time, and
unroll partially if the trip count is not known at compile time. This differs from
"llvm.loop.unroll.full" which explicitly does not unroll a loop if the trip count is not
known at compile time

With this change "#pragma unroll" generates "llvm.loop.unroll.enable" rather than
"llvm.loop.unroll.full" metadata. This changes the semantics of "#pragma unroll" slightly
to mean "unroll aggressively (fully or partially)" rather than "unroll fully or not at all".

The motivating example for this change was some internal code with a loop marked
with "#pragma unroll" which only sometimes had a compile-time trip count depending
on template magic. When the trip count was a compile-time constant, everything works
as expected and the loop is fully unrolled. However, when the trip count was not a
compile-time constant the "#pragma unroll" explicitly disabled unrolling of the loop(!).
Removing "#pragma unroll" caused the loop to be unrolled partially which was desirable
from a performance perspective.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244467 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 16 insertions, 4 deletions

diff --git a/docs/LanguageExtensions.rst b/docs/LanguageExtensions.rst
index 88e42e95ba..8cc0f98eb7 100644
--- a/docs/LanguageExtensions.rst
+++ b/docs/LanguageExtensions.rst
@@ -1993,11 +1993,23 @@ iterations. Full unrolling is only possible if the loop trip count is known at
 compile time. Partial unrolling replicates the loop body within the loop and
 reduces the trip count.
 
-If ``unroll(full)`` is specified the unroller will attempt to fully unroll the
+If ``unroll(enable)`` is specified the unroller will attempt to fully unroll the
 loop if the trip count is known at compile time. If the fully unrolled code size
 is greater than an internal limit the loop will be partially unrolled up to this
-limit. If the loop count is not known at compile time the loop will not be
-unrolled.
+limit. If the trip count is not known at compile time the loop will be partially
+unrolled with a heuristically chosen unroll factor.
+
+.. code-block:: c++
+
+  #pragma clang loop unroll(enable)
+  for(...) {
+    ...
+  }
+
+If ``unroll(full)`` is specified the unroller will attempt to fully unroll the
+loop if the trip count is known at compile time identically to
+``unroll(enable)``. However, with ``unroll(full)`` the loop will not be unrolled
+if the loop count is not known at compile time.
 
 .. code-block:: c++
 
@@ -2009,7 +2021,7 @@ unrolled.
 The unroll count can be specified explicitly with ``unroll_count(_value_)`` where
 _value_ is a positive integer. If this value is greater than the trip count the
 loop will be fully unrolled. Otherwise the loop is partially unrolled subject
-to the same code size limit as with ``unroll(full)``.
+to the same code size limit as with ``unroll(enable)``.
 
 .. code-block:: c++