Improve LangRef.rst entry

Author: jdenny-ornl

llvm/docs/LangRef.rst

@@ -7869,13 +7869,31 @@ loop distribution pass. See
 '``llvm.loop.estimated_trip_count``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-This metadata records the loop's estimated trip count.  If it is not present, a
-loop's estimated trip count should be computed from any ``branch_weights``
-metadata attached to the latch block's branch instruction.
+This metadata records the loop's estimated trip count.  The first
+operand is the string ``llvm.loop.estimated_trip_count`` and the
+second operand is an integer specifying the count.  For example:
 
-Thus, this metadata frees loop transformations to compute latch branch weights
-solely for the purpose of maintaining accurate block frequencies instead of
-requiring the branch weights to always serve both roles.
+.. code-block:: llvm
+
+   !0 = !{!"llvm.loop.estimated_trip_count", i32 8}
+
+A loop's estimated trip count is an estimate of the average number of
+loop iterations (specifically, the number of times the loop's header
+executes) each time execution reaches the loop.  It is usually only an
+estimate based on, for example, profile data.  The actual number of
+iterations might vary widely.
+
+The estimated trip count serves as a parameter for various loop
+transformations and typically helps estimate transformation cost.  For
+example, it can help determine how many iterations to peel or how
+aggressively to unroll.
+
+If this metadata is not present, such passes compute the estimated
+trip count from any ``branch_weights`` metadata attached to the latch
+block's branch instruction.  Thus, this metadata frees loop
+transformations to compute latch branch weights solely for the purpose
+of maintaining accurate block frequencies instead of requiring the
+branch weights to always serve both roles.
 
 '``llvm.licm.disable``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^