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

[Eli Friedman via llvm-commits]

================
@@ -27666,6 +27666,119 @@ 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 the allocated object. It must be used in combination with
+:ref:`llvm.can.load.speculatively <int_can_load_speculatively>` to ensure
+the access cannot fault.
+
+For bytes that are within the bounds of the allocated object, the intrinsic
+returns the stored value. For bytes that are beyond the bounds of the
+allocated object, the intrinsic returns ``undef`` for those bytes. At least the
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efriedma-quic wrote:

Maybe consider using poison here?  Not sure.

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