Allow shifts with constant signed positive amounts (p4lang#3303)

* Allow shifts with constant signed positive amounts
Signed-off-by: Mihai Budiu <[email protected]>

backends/bmv2/common/lower.cpp

@@ -141,7 +141,6 @@ const IR::Node* LowerExpressions::postorder(IR::Concat* expression) {
     BUG_CHECK(resulttype->is<IR::Type_Bits>(), "%1%: expected a bitstring got a %2%",
               expression->right, type);
     unsigned sizeofb = type->to<IR::Type_Bits>()->size;
-    unsigned sizeofresult = resulttype->to<IR::Type_Bits>()->size;
     auto cast0 = new IR::Cast(expression->left->srcInfo, resulttype, expression->left);
     auto cast1 = new IR::Cast(expression->right->srcInfo, resulttype, expression->right);
 

frontends/common/constantFolding.cpp

@@ -777,12 +777,6 @@ const IR::Node* DoConstantFolding::shift(const IR::Operation_Binary* e) {
         ::error(ErrorType::ERR_INVALID, "%1%: Shifts with negative amounts are not permitted", e);
         return e;
     }
-    if (auto crTypeBits = cr->type->to<IR::Type_Bits>()) {
-        if (crTypeBits->isSigned) {
-            ::error(ErrorType::ERR_EXPECTED, "%1%: shift amounts cannot be signed", right);
-            return e;
-        }
-    }
 
     if (cr->value == 0) {
         // ::warning("%1% with zero", e);

frontends/p4/typeChecking/typeChecker.cpp

@@ -2301,8 +2301,7 @@ const IR::Node* TypeInference::shift(const IR::Operation_Binary* expression) {
         return expression;
     }
     auto lt = ltype->to<IR::Type_Bits>();
-    if (expression->right->is<IR::Constant>()) {
-        auto cst = expression->right->to<IR::Constant>();
+    if (auto cst = expression->right->to<IR::Constant>()) {
         if (!cst->fitsInt()) {
             typeError("Shift amount too large: %1%", cst);
             return expression;
@@ -2315,6 +2314,20 @@ const IR::Node* TypeInference::shift(const IR::Operation_Binary* expression) {
         if (lt != nullptr && shift >= lt->size)
             warn(ErrorType::WARN_OVERFLOW, "%1%: shifting value with %2% bits by %3%",
                  expression, lt->size, shift);
+        // If the amount is signed but positive, make it unsigned
+        if (auto bt = rtype->to<IR::Type_Bits>()) {
+            if (bt->isSigned) {
+                rtype = new IR::Type_Bits(rtype->srcInfo, bt->width_bits(), false);
+                auto amt = new IR::Constant(cst->srcInfo, rtype, cst->value, cst->base);
+                if (expression->is<IR::Shl>()) {
+                    expression = new IR::Shl(expression->srcInfo, expression->left, amt);
+                } else {
+                    expression = new IR::Shr(expression->srcInfo, expression->left, amt);
+                }
+                setCompileTimeConstant(expression->right);
+                setType(expression->right, rtype);
+            }
+        }
     }
 
     if (rtype->is<IR::Type_Bits>() && rtype->to<IR::Type_Bits>()->isSigned) {

testdata/p4_16_samples/issue3287.p4

@@ -0,0 +1,19 @@
+#include <core.p4>
+
+bit<4> func(in bit<4> l)
+{
+  const int<6> tt = 1;
+  return l << tt;
+}
+
+parser p(out bit<4> result) {
+    state start {
+        result = func(1);
+        transition accept;
+    }
+}
+
+parser P(out bit<4> r);
+package top(P p);
+
+top(p()) main;

testdata/p4_16_samples_outputs/issue3287-first.p4

@@ -0,0 +1,17 @@
+#include <core.p4>
+
+bit<4> func(in bit<4> l) {
+    const int<6> tt = 6s1;
+    return l << 6w1;
+}
+parser p(out bit<4> result) {
+    state start {
+        result = func(4w1);
+        transition accept;
+    }
+}
+
+parser P(out bit<4> r);
+package top(P p);
+top(p()) main;
+

testdata/p4_16_samples_outputs/issue3287-frontend.p4

@@ -0,0 +1,20 @@
+#include <core.p4>
+
+parser p(out bit<4> result) {
+    @name("p.l_0") bit<4> l;
+    @name("p.hasReturned") bool hasReturned;
+    @name("p.retval") bit<4> retval;
+    state start {
+        l = 4w1;
+        hasReturned = false;
+        hasReturned = true;
+        retval = l << 6w1;
+        result = retval;
+        transition accept;
+    }
+}
+
+parser P(out bit<4> r);
+package top(P p);
+top(p()) main;
+

testdata/p4_16_samples_outputs/issue3287-midend.p4

@@ -0,0 +1,13 @@
+#include <core.p4>
+
+parser p(out bit<4> result) {
+    state start {
+        result = 4w2;
+        transition accept;
+    }
+}
+
+parser P(out bit<4> r);
+package top(P p);
+top(p()) main;
+

testdata/p4_16_samples_outputs/issue3287.p4

@@ -0,0 +1,17 @@
+#include <core.p4>
+
+bit<4> func(in bit<4> l) {
+    const int<6> tt = 1;
+    return l << tt;
+}
+parser p(out bit<4> result) {
+    state start {
+        result = func(1);
+        transition accept;
+    }
+}
+
+parser P(out bit<4> r);
+package top(P p);
+top(p()) main;
+