[llvm] 7193f72 - docs: Emphasize ArrayRef over SmallVectorImpl

[Duncan P. N. Exon Smith via llvm-commits]

Author: Duncan P. N. Exon Smith
Date: 2020-10-06T18:13:52-04:00
New Revision: 7193f727988360acb7037b42851f0a6fae29af9b

URL: https://github.com/llvm/llvm-project/commit/7193f727988360acb7037b42851f0a6fae29af9b
DIFF: https://github.com/llvm/llvm-project/commit/7193f727988360acb7037b42851f0a6fae29af9b.diff

LOG: docs: Emphasize ArrayRef over SmallVectorImpl

The section on SmallVector has a note about preferring SmallVectorImpl
for APIs but doesn't mention ArrayRef. Although ArrayRef is discussed
elsewhere, let's re-emphasize here.

Differential Revision: https://reviews.llvm.org/D49881

Added: 
    

Modified: 
    llvm/docs/ProgrammersManual.rst

Removed: 
    


################################################################################
diff  --git a/llvm/docs/ProgrammersManual.rst b/llvm/docs/ProgrammersManual.rst
index 8d028a64b65f..d9925d69d9f6 100644
--- a/llvm/docs/ProgrammersManual.rst
+++ b/llvm/docs/ProgrammersManual.rst
@@ -1541,30 +1541,43 @@ SmallVector has grown a few other minor advantages over std::vector, causing
 
 .. note::
 
-   Prefer to use ``SmallVectorImpl<T>`` as a parameter type.
+   Prefer to use ``ArrayRef<T>`` or ``SmallVectorImpl<T>`` as a parameter type.
 
-   In APIs that don't care about the "small size" (most?), prefer to use
-   the ``SmallVectorImpl<T>`` class, which is basically just the "vector
-   header" (and methods) without the elements allocated after it. Note that
-   ``SmallVector<T, N>`` inherits from ``SmallVectorImpl<T>`` so the
-   conversion is implicit and costs nothing. E.g.
+   It's rarely appropriate to use ``SmallVector<T, N>`` as a parameter type.
+   If an API only reads from the vector, it should use :ref:`ArrayRef
+   <dss_arrayref>`.  Even if an API updates the vector the "small size" is
+   unlikely to be relevant; such an API should use the ``SmallVectorImpl<T>``
+   class, which is the "vector header" (and methods) without the elements
+   allocated after it. Note that ``SmallVector<T, N>`` inherits from
+   ``SmallVectorImpl<T>`` so the conversion is implicit and costs nothing. E.g.
 
    .. code-block:: c++
 
-      // BAD: Clients cannot pass e.g. SmallVector<Foo, 4>.
+      // DISCOURAGED: Clients cannot pass e.g. raw arrays.
+      hardcodedContiguousStorage(const SmallVectorImpl<Foo> &In);
+      // ENCOURAGED: Clients can pass any contiguous storage of Foo.
+      allowsAnyContiguousStorage(ArrayRef<Foo> In);
+
+      void someFunc1() {
+        Foo Vec[] = { /* ... */ };
+        hardcodedContiguousStorage(Vec); // Error.
+        allowsAnyContiguousStorage(Vec); // Works.
+      }
+
+      // DISCOURAGED: Clients cannot pass e.g. SmallVector<Foo, 8>.
       hardcodedSmallSize(SmallVector<Foo, 2> &Out);
-      // GOOD: Clients can pass any SmallVector<Foo, N>.
+      // ENCOURAGED: Clients can pass any SmallVector<Foo, N>.
       allowsAnySmallSize(SmallVectorImpl<Foo> &Out);
 
-      void someFunc() {
+      void someFunc2() {
         SmallVector<Foo, 8> Vec;
         hardcodedSmallSize(Vec); // Error.
         allowsAnySmallSize(Vec); // Works.
       }
 
-   Even though it has "``Impl``" in the name, this is so widely used that
-   it really isn't "private to the implementation" anymore. A name like
-   ``SmallVectorHeader`` would be more appropriate.
+   Even though it has "``Impl``" in the name, SmallVectorImpl is widely used
+   and is no longer "private to the implementation". A name like
+   ``SmallVectorHeader`` might be more appropriate.
 
 .. _dss_vector: