[llvm] Provide intrinsics for speculative loads (PR #179642)

[Florian Hahn via llvm-commits]

================
@@ -27692,6 +27692,127 @@ The '``llvm.masked.compressstore``' intrinsic is designed for compressing data i
 Other targets may support this intrinsic differently, for example, by lowering it into a sequence of branches that guard scalar store operations.
 
 
+Speculative Load Intrinsics
+---------------------------
+
+LLVM provides intrinsics for speculatively loading memory that may be
+out-of-bounds. These intrinsics enable optimizations like early-exit loop
+vectorization where the vectorized loop may read beyond the end of an array,
+provided the access is guaranteed to not trap by target-specific checks.
+
+.. _int_speculative_load:
+
+'``llvm.speculative.load``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+This is an overloaded intrinsic.
+
+::
+
+      declare <4 x float>  @llvm.speculative.load.v4f32.p0(ptr <ptr>)
+      declare <8 x i32>    @llvm.speculative.load.v8i32.p0(ptr <ptr>)
+      declare i64          @llvm.speculative.load.i64.p0(ptr <ptr>)
+
+Overview:
+"""""""""
+
+The '``llvm.speculative.load``' intrinsic loads a value from memory. Unlike a
+regular load, the memory access may extend beyond the bounds of the allocated
+object, provided the pointer has been verified by
+:ref:`llvm.can.load.speculatively <int_can_load_speculatively>` to ensure the
+access cannot fault.
+
+Arguments:
+""""""""""
+
+The argument is a pointer to the memory location to load from. The return type
+must have a power-of-2 size in bytes.
+
+Semantics:
+""""""""""
+
+The '``llvm.speculative.load``' intrinsic performs a load that may access
+memory beyond what is accessible through the pointer. It must be used in
+combination with :ref:`llvm.can.load.speculatively <int_can_load_speculatively>`
+to ensure the access can be performed speculatively.
+
+A byte at ``ptr + i`` is *accessible through* ``ptr`` if both of the following
+hold:
+
+1. The byte lies within the bounds of an allocated object that ``ptr`` is
+   :ref:`based <pointeraliasing>` on.
+2. Accessing the byte through ``ptr`` does not violate any ``noalias``
+   constraints.
----------------
fhahn wrote:

Updated to now always pass in the accessed bytes, as per the RFC discussion. WDYT

https://github.com/llvm/llvm-project/pull/179642