From 05e10e917a46233822c5c7c58b3ac1f29850c2d8 Mon Sep 17 00:00:00 2001
From: Chris Dodd <cdodd@nvidia.com>
Date: Thu, 12 Dec 2024 02:00:19 +0000
Subject: [PATCH] Remove @likely/@unlikely when constfolding if statements

- warn if @unlikely is always taken.
---
 frontends/common/constantFolding.cpp          | 25 ++++++++++++++++--
 testdata/p4_16_samples/annotation-likely.p4   | 22 ++++++++++++++++
 .../annotation-likely-first.p4                | 17 ++++++++++++
 .../annotation-likely-frontend.p4             | 17 ++++++++++++
 .../annotation-likely-midend.p4               | 26 +++++++++++++++++++
 .../annotation-likely.p4                      | 21 +++++++++++++++
 .../annotation-likely.p4-stderr               |  3 +++
 7 files changed, 129 insertions(+), 2 deletions(-)
 create mode 100644 testdata/p4_16_samples/annotation-likely.p4
 create mode 100644 testdata/p4_16_samples_outputs/annotation-likely-first.p4
 create mode 100644 testdata/p4_16_samples_outputs/annotation-likely-frontend.p4
 create mode 100644 testdata/p4_16_samples_outputs/annotation-likely-midend.p4
 create mode 100644 testdata/p4_16_samples_outputs/annotation-likely.p4
 create mode 100644 testdata/p4_16_samples_outputs/annotation-likely.p4-stderr

diff --git a/frontends/common/constantFolding.cpp b/frontends/common/constantFolding.cpp
index 2ba8c18ae94..35b14dd0166 100644
--- a/frontends/common/constantFolding.cpp
+++ b/frontends/common/constantFolding.cpp
@@ -1108,15 +1108,36 @@ const IR::Node *DoConstantFolding::postorder(IR::SelectExpression *expression) {
     return result;
 }
 
+class FilterLikelyAnnot : public Transform {
+    IR::Statement *preorder(IR::Statement *stmt) {
+        prune();
+        return stmt;
+    }
+    IR::BlockStatement *preorder(IR::BlockStatement *stmt) {
+        prune();
+        visit(stmt->annotations, "annotations");
+        return stmt;
+    }
+    IR::Annotation *preorder(IR::Annotation *annot) {
+        prune();
+        if (annot->name == IR::Annotation::likelyAnnotation) return nullptr;
+        if (annot->name == IR::Annotation::unlikelyAnnotation) {
+            warning(ErrorType::WARN_IGNORE, "ignoring %1% on always taken statement", annot);
+            return nullptr;
+        }
+        return annot;
+    }
+};
+
 const IR::Node *DoConstantFolding::postorder(IR::IfStatement *ifstmt) {
     if (auto cond = ifstmt->condition->to<IR::BoolLiteral>()) {
         if (cond->value) {
-            return ifstmt->ifTrue;
+            return ifstmt->ifTrue->apply(FilterLikelyAnnot());
         } else {
             if (ifstmt->ifFalse == nullptr) {
                 return new IR::EmptyStatement(ifstmt->srcInfo);
             } else {
-                return ifstmt->ifFalse;
+                return ifstmt->ifFalse->apply(FilterLikelyAnnot());
             }
         }
     }
diff --git a/testdata/p4_16_samples/annotation-likely.p4 b/testdata/p4_16_samples/annotation-likely.p4
new file mode 100644
index 00000000000..ba9726f6c8a
--- /dev/null
+++ b/testdata/p4_16_samples/annotation-likely.p4
@@ -0,0 +1,22 @@
+#include <core.p4>
+
+struct Headers {
+    bit<8> a;
+    bit<8> b;
+}
+
+control ingress(inout Headers h) {
+    apply {
+        if (true) @likely {
+            h.a = 0;
+        }
+        if (true) @unlikely {
+            h.b = 0;
+        }
+    }
+}
+
+control c<T>(inout T d);
+package top<T>(c<T> _c);
+
+top(ingress()) main;
diff --git a/testdata/p4_16_samples_outputs/annotation-likely-first.p4 b/testdata/p4_16_samples_outputs/annotation-likely-first.p4
new file mode 100644
index 00000000000..8661c2d5bea
--- /dev/null
+++ b/testdata/p4_16_samples_outputs/annotation-likely-first.p4
@@ -0,0 +1,17 @@
+#include <core.p4>
+
+struct Headers {
+    bit<8> a;
+    bit<8> b;
+}
+
+control ingress(inout Headers h) {
+    apply {
+        h.a = 8w0;
+        h.b = 8w0;
+    }
+}
+
+control c<T>(inout T d);
+package top<T>(c<T> _c);
+top<Headers>(ingress()) main;
diff --git a/testdata/p4_16_samples_outputs/annotation-likely-frontend.p4 b/testdata/p4_16_samples_outputs/annotation-likely-frontend.p4
new file mode 100644
index 00000000000..8661c2d5bea
--- /dev/null
+++ b/testdata/p4_16_samples_outputs/annotation-likely-frontend.p4
@@ -0,0 +1,17 @@
+#include <core.p4>
+
+struct Headers {
+    bit<8> a;
+    bit<8> b;
+}
+
+control ingress(inout Headers h) {
+    apply {
+        h.a = 8w0;
+        h.b = 8w0;
+    }
+}
+
+control c<T>(inout T d);
+package top<T>(c<T> _c);
+top<Headers>(ingress()) main;
diff --git a/testdata/p4_16_samples_outputs/annotation-likely-midend.p4 b/testdata/p4_16_samples_outputs/annotation-likely-midend.p4
new file mode 100644
index 00000000000..3c46706ac0f
--- /dev/null
+++ b/testdata/p4_16_samples_outputs/annotation-likely-midend.p4
@@ -0,0 +1,26 @@
+#include <core.p4>
+
+struct Headers {
+    bit<8> a;
+    bit<8> b;
+}
+
+control ingress(inout Headers h) {
+    @hidden action annotationlikely11() {
+        h.a = 8w0;
+        h.b = 8w0;
+    }
+    @hidden table tbl_annotationlikely11 {
+        actions = {
+            annotationlikely11();
+        }
+        const default_action = annotationlikely11();
+    }
+    apply {
+        tbl_annotationlikely11.apply();
+    }
+}
+
+control c<T>(inout T d);
+package top<T>(c<T> _c);
+top<Headers>(ingress()) main;
diff --git a/testdata/p4_16_samples_outputs/annotation-likely.p4 b/testdata/p4_16_samples_outputs/annotation-likely.p4
new file mode 100644
index 00000000000..4ea67d25908
--- /dev/null
+++ b/testdata/p4_16_samples_outputs/annotation-likely.p4
@@ -0,0 +1,21 @@
+#include <core.p4>
+
+struct Headers {
+    bit<8> a;
+    bit<8> b;
+}
+
+control ingress(inout Headers h) {
+    apply {
+        if (true) @likely {
+            h.a = 0;
+        }
+        if (true) @unlikely {
+            h.b = 0;
+        }
+    }
+}
+
+control c<T>(inout T d);
+package top<T>(c<T> _c);
+top(ingress()) main;
diff --git a/testdata/p4_16_samples_outputs/annotation-likely.p4-stderr b/testdata/p4_16_samples_outputs/annotation-likely.p4-stderr
new file mode 100644
index 00000000000..fe7c594fa6a
--- /dev/null
+++ b/testdata/p4_16_samples_outputs/annotation-likely.p4-stderr
@@ -0,0 +1,3 @@
+annotation-likely.p4(13): [--Wwarn=ignore] warning: ignoring @unlikely on always taken statement
+        if (true) @unlikely {
+                  ^