interp: add support for reading a pointer tag

This is necessary to get #3691
working.

Author: aykevl

interp/interpreter.go

@@ -746,6 +746,14 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 					// src/strings/builder.go in the Go source tree. This is
 					// the identity operator, so we can return the input.
 					locals[inst.localIndex] = lhs
+				} else if inst.opcode == llvm.And && rhs.Uint() < 8 {
+					// This is probably part of a pattern to get the lower bits
+					// of a pointer for pointer tagging, like this:
+					//     uintptr(unsafe.Pointer(t)) & 0b11
+					// We can actually support this easily by ANDing with the
+					// pointer offset.
+					result := uint64(lhsPtr.offset()) & rhs.Uint()
+					locals[inst.localIndex] = makeLiteralInt(result, int(lhs.len(r)*8))
 				} else {
 					// Catch-all for weird operations that should just be done
 					// at runtime.

interp/testdata/consteval.ll

@@ -4,6 +4,7 @@ target triple = "x86_64--linux"
 @intToPtrResult = global i8 0
 @ptrToIntResult = global i8 0
 @icmpResult = global i8 0
+@pointerTagResult = global i64 0
 @someArray = internal global {i16, i8, i8} zeroinitializer
 @someArrayPointer = global ptr zeroinitializer
 
@@ -17,6 +18,7 @@ define internal void @main.init() {
   call void @testPtrToInt()
   call void @testConstGEP()
   call void @testICmp()
+  call void @testPointerTag()
   ret void
 }
 
@@ -63,3 +65,9 @@ unequal:
   ret void
   ret void
 }
+
+define internal void @testPointerTag() {
+  %val = and i64 ptrtoint (ptr getelementptr inbounds (i8, ptr @someArray, i32 2) to i64), 3
+  store i64 %val, ptr @pointerTagResult
+  ret void
+}

interp/testdata/consteval.out.ll

@@ -4,6 +4,7 @@ target triple = "x86_64--linux"
 @intToPtrResult = local_unnamed_addr global i8 2
 @ptrToIntResult = local_unnamed_addr global i8 2
 @icmpResult = local_unnamed_addr global i8 2
+@pointerTagResult = local_unnamed_addr global i64 2
 @someArray = internal global { i16, i8, i8 } zeroinitializer
 @someArrayPointer = local_unnamed_addr global ptr getelementptr inbounds ({ i16, i8, i8 }, ptr @someArray, i64 0, i32 1)