[CodeGen] add/fix rotate builtins that map to LLVM funnel shift (retry)

This is a retry of rL340135 (reverted at rL340136 because of gcc host compiler crashing)
with 2 changes:
1. Move the code into a helper to reduce code duplication (and hopefully work-around the crash).
2. The original commit had a formatting bug in the docs (missing an underscore).

Original commit message:

This exposes the LLVM funnel shift intrinsics as more familiar bit rotation functions in clang
(when both halves of a funnel shift are the same value, it's a rotate).

We're free to name these as we want because we're not copying gcc, but if there's some other
existing art (eg, the microsoft ops that are modified in this patch) that we want to replicate,
we can change the names.

The funnel shift intrinsics were added here:
https://reviews.llvm.org/D49242

With improved codegen in:
https://reviews.llvm.org/rL337966
https://reviews.llvm.org/rL339359

And basic IR optimization added in:
https://reviews.llvm.org/rL338218
https://reviews.llvm.org/rL340022

...so these are expected to produce asm output that's equal or better to the multi-instruction
alternatives using primitive C/IR ops.

In the motivating loop example from PR37387:
https://bugs.llvm.org/show_bug.cgi?id=37387#c7
...we get the expected 'rolq' x86 instructions if we substitute the rotate builtin into the source.

Differential Revision: https://reviews.llvm.org/D50924

llvm-svn: 340137

1 files changed, 64 insertions, 0 deletions

diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index 1aef265a8589..894b7504fd2c 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -1739,6 +1739,70 @@ The '``__builtin_bitreverse``' family of builtins is used to reverse
 the bitpattern of an integer value; for example ``0b10110110`` becomes
 ``0b01101101``.
 
+``__builtin_rotateleft``
+------------------------
+
+* ``__builtin_rotateleft8``
+* ``__builtin_rotateleft16``
+* ``__builtin_rotateleft32``
+* ``__builtin_rotateleft64``
+
+**Syntax**:
+
+.. code-block:: c++
+
+     __builtin_rotateleft32(x, y)
+
+**Examples**:
+
+.. code-block:: c++
+
+      uint8_t rot_x = __builtin_rotateleft8(x, y);
+      uint16_t rot_x = __builtin_rotateleft16(x, y);
+      uint32_t rot_x = __builtin_rotateleft32(x, y);
+      uint64_t rot_x = __builtin_rotateleft64(x, y);
+
+**Description**:
+
+The '``__builtin_rotateleft``' family of builtins is used to rotate
+the bits in the first argument by the amount in the second argument. 
+For example, ``0b10000110`` rotated left by 11 becomes ``0b00110100``.
+The shift value is treated as an unsigned amount modulo the size of
+the arguments. Both arguments and the result have the bitwidth specified
+by the name of the builtin.
+
+``__builtin_rotateright``
+_------------------------
+
+* ``__builtin_rotateright8``
+* ``__builtin_rotateright16``
+* ``__builtin_rotateright32``
+* ``__builtin_rotateright64``
+
+**Syntax**:
+
+.. code-block:: c++
+
+     __builtin_rotateright32(x, y)
+
+**Examples**:
+
+.. code-block:: c++
+
+      uint8_t rot_x = __builtin_rotateright8(x, y);
+      uint16_t rot_x = __builtin_rotateright16(x, y);
+      uint32_t rot_x = __builtin_rotateright32(x, y);
+      uint64_t rot_x = __builtin_rotateright64(x, y);
+
+**Description**:
+
+The '``__builtin_rotateright``' family of builtins is used to rotate
+the bits in the first argument by the amount in the second argument. 
+For example, ``0b10000110`` rotated right by 3 becomes ``0b11010000``.
+The shift value is treated as an unsigned amount modulo the size of
+the arguments. Both arguments and the result have the bitwidth specified
+by the name of the builtin.
+
 ``__builtin_unreachable``
 -------------------------