ssa cfg builder: treat constant switches explicitly

Author: clonker

libyul/backends/evm/SSAControlFlowGraphBuilder.cpp

@@ -355,6 +355,28 @@ void SSAControlFlowGraphBuilder::operator()(Switch const& _switch)
 	}
 	else
 	{
+		if (auto const* constantExpression = std::get_if<Literal>(_switch.expression.get()))
+		{
+			Case const* matchedCase = nullptr;
+			// select case that matches (or default if available)
+			for (auto const& switchCase: _switch.cases)
+			{
+				if (!switchCase.value)
+					matchedCase = &switchCase;
+				if (switchCase.value && switchCase.value->value.value() == constantExpression->value.value())
+				{
+					matchedCase = &switchCase;
+					break;
+				}
+			}
+			if (matchedCase)
+			{
+				// inject directly into the current block
+				(*this)(matchedCase->body);
+			}
+			return;
+		}
+
 		std::optional<BuiltinHandle> equalityBuiltinHandle = m_dialect.equalityFunctionHandle();
 		yulAssert(equalityBuiltinHandle);
 

test/libyul/yulSSAControlFlowGraph/switch_const.yul

@@ -0,0 +1,55 @@
+{
+    let x := calldataload(3)
+
+    // this should yield calldataload(88) directly
+    switch 1
+    case 0 {
+        x := calldataload(77)
+    }
+    case 1 {
+        x := calldataload(88)
+    }
+    default {
+        x := calldataload(99)
+    }
+
+    // this should yield the default case
+    switch 55
+    case 0 {
+        x := calldataload(77)
+    }
+    case 1 {
+        x := calldataload(88)
+    }
+    default {
+        x := calldataload(99)
+    }
+
+    // this should be skipped entirely
+    switch 66
+    case 0 {
+        x := calldataload(77)
+    }
+    case 1 {
+        x := calldataload(88)
+    }
+    sstore(x, 0)
+}
+// ----
+// digraph SSACFG {
+// nodesep=0.7;
+// graph[fontname="DejaVu Sans"]
+// node[shape=box,fontname="DejaVu Sans"];
+//
+// Entry0 [label="Entry"];
+// Entry0 -> Block0_0;
+// Block0_0 [label="\
+// Block 0; (0, max 0)\nLiveIn: \l\
+// LiveOut: \l\nv1 := calldataload(3)\l\
+// v4 := calldataload(88)\l\
+// v7 := calldataload(99)\l\
+// sstore(0, v7)\l\
+// "];
+// Block0_0Exit [label="MainExit"];
+// Block0_0 -> Block0_0Exit;
+// }