[llvm-branch-commits] [IR] Introduce `llvm.experimental.hot()` (PR #84850)

[via llvm-branch-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Vitaly Buka (vitalybuka)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/84850.diff


4 Files Affected:

- (modified) llvm/docs/LangRef.rst (+48) 
- (modified) llvm/include/llvm/IR/Intrinsics.td (+5) 
- (modified) llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp (+6) 
- (added) llvm/test/CodeGen/Generic/builtin-hot.ll (+19) 


``````````diff
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index d613ceea8654f8..36f4c964ee296c 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -27639,6 +27639,54 @@ constant `true`. However it is always correct to replace
 it with any other `i1` value. Any pass can
 freely do it if it can benefit from non-default lowering.
 
+'``llvm.experimental.hot``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare i1 @llvm.experimental.hot()
+
+Overview:
+"""""""""
+
+This intrinsic returns true iff it's known that containing basic block is hot in
+profile.
+
+When used with profile based optimization allows to change program behaviour
+deppending on the code hotness.
+
+Arguments:
+""""""""""
+
+None.
+
+Semantics:
+""""""""""
+
+The intrinsic ``@llvm.experimental.hot()`` returns either `true` or `false`,
+deppending on profile used. Expresion is evaluated as `true` iff profile and
+summary are availible and profile counter for the block reach hotness threshold.
+For each evaluation of a call to this intrinsic, the program must be valid and
+correct both if it returns `true` and if it returns `false`.
+
+When used in a branch condition, it allows us to choose between
+two alternative correct solutions for the same problem, like
+in example below:
+
+.. code-block:: text
+
+    %cond = call i1 @llvm.experimental.hot()
+    br i1 %cond, label %fast_path, label %slow_path
+
+  label %fast_path:
+    ; Omit diagnostics.
+
+  label %slow_path:
+    ; Additional diagnostics.
+
 
 '``llvm.load.relative``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/llvm/include/llvm/IR/Intrinsics.td b/llvm/include/llvm/IR/Intrinsics.td
index 144298fd7c0162..96e9cdf6627a75 100644
--- a/llvm/include/llvm/IR/Intrinsics.td
+++ b/llvm/include/llvm/IR/Intrinsics.td
@@ -1722,6 +1722,11 @@ def int_debugtrap : Intrinsic<[]>,
 def int_ubsantrap : Intrinsic<[], [llvm_i8_ty],
                               [IntrNoReturn, IntrCold, ImmArg<ArgIndex<0>>]>;
 
+// Return true if profile counter for containing block is hot.
+def int_experimental_hot : Intrinsic<[llvm_i1_ty], [],
+                                      [IntrInaccessibleMemOnly, IntrWriteMem,
+                                       IntrWillReturn, NoUndef<RetIndex>]>;
+
 // Support for dynamic deoptimization (or de-specialization)
 def int_experimental_deoptimize : Intrinsic<[llvm_any_ty], [llvm_vararg_ty],
                                             [Throws]>;
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 22e57d0d99e9b1..8e73433ce82ea5 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -7276,6 +7276,12 @@ void SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I,
     setValue(&I, getValue(I.getArgOperand(0)));
     return;
 
+  case Intrinsic::experimental_hot:
+    // Default lowering to false. It's intended to be lowered as soon as profile
+    // is avalible to unblock other optimizations.
+    setValue(&I, DAG.getConstant(0, sdl, MVT::i1));
+    return;
+
   case Intrinsic::ubsantrap:
   case Intrinsic::debugtrap:
   case Intrinsic::trap: {
diff --git a/llvm/test/CodeGen/Generic/builtin-hot.ll b/llvm/test/CodeGen/Generic/builtin-hot.ll
new file mode 100644
index 00000000000000..449f58d3c00675
--- /dev/null
+++ b/llvm/test/CodeGen/Generic/builtin-hot.ll
@@ -0,0 +1,19 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
+; RUN: llc -o - %s | FileCheck %s
+
+; REQUIRES: aarch64-registered-target
+
+target triple = "aarch64-linux"
+
+define i1 @test()  {
+; CHECK-LABEL: test:
+; CHECK:       // %bb.0: // %entry
+; CHECK-NEXT:    mov w0, wzr
+; CHECK-NEXT:    ret
+entry:
+  %hot = call i1 @llvm.experimental.hot()
+  ret i1 %hot
+}
+
+declare i1 @llvm.expect.hot() nounwind
+

``````````

</details>


https://github.com/llvm/llvm-project/pull/84850