Bug fix for lossless_cast with minor additions

src/IROperator.cpp

@@ -11,6 +11,7 @@
 #include "IRMutator.h"
 #include "IROperator.h"
 #include "IRPrinter.h"
+#include "Simplify.h"
 #include "Util.h"
 #include "Var.h"
 
@@ -446,8 +447,9 @@ Expr lossless_cast(Type t, Expr e) {
     }
 
     if (const Cast *c = e.as<Cast>()) {
-        if (t.can_represent(c->value.type())) {
-            // We can recurse into widening casts.
+        // We can recurse into widening casts.
+        if (c->type.can_represent(c->value.type()) &&
+            t.can_represent(c->value.type())) {
             return lossless_cast(t, c->value);
         } else {
             return Expr();
@@ -537,6 +539,7 @@ Expr lossless_cast(Type t, Expr e) {
                     Type narrower = reduce->value.type().with_bits(t.bits() / 2);
                     Expr val = lossless_cast(narrower, reduce->value);
                     if (val.defined()) {
+                        val = cast(narrower.with_bits(t.bits()), val);
                         return VectorReduce::make(reduce->op, val, reduce->type.lanes());
                     }
                 }
@@ -566,6 +569,42 @@ Expr lossless_cast(Type t, Expr e) {
         return Shuffle::make(vecs, shuf->indices);
     }
 
+    if (const Mod *mod = e.as<Mod>()) {
+        Expr val;
+        if (e.type().is_uint()) {
+            val = simplify(mod->b + make_const(e.type(), -1));
+        } else if (e.type().is_int()) {
+            // 0 <= a%b < |b|
+            Type wide_ty = e.type().with_bits(e.type().bits() * 2);
+            Expr b = cast(wide_ty, mod->b);
+            Expr minus_one = make_const(wide_ty, -1);
+            val = simplify(Max::make(b, minus_one * b) + minus_one);
+        }
+        if (lossless_cast(t, val).defined()) {
+            return cast(t, e);
+        } else {
+            return Expr();
+        }
+    }
+
+    if (const Ramp *ramp = e.as<Ramp>()) {
+        if (t.bits() > 32) {
+            return Expr();
+        }
+        Type ty = t.with_lanes(ramp->type.lanes() / t.lanes());
+        Type wide_ty = ty.with_bits(64);
+        Expr first = ramp->base;
+        // Cast to wide_ty to prevent overflows.
+        Expr last = simplify(cast(wide_ty, first) +
+                             cast(wide_ty, ramp->lanes - 1) * cast(wide_ty, ramp->stride));
+        if (lossless_cast(ty, first).defined() &&
+            lossless_cast(ty, last).defined()) {
+            return cast(t, e);
+        } else {
+            return Expr();
+        }
+    }
+
     return Expr();
 }
 
@@ -2365,4 +2404,83 @@ Expr undef(Type t) {
                                 Internal::Call::PureIntrinsic);
 }
 
+namespace Internal {
+
+void check_lossless_cast(const Type &t, const Expr &in, const Expr &correct) {
+    Expr result = lossless_cast(t, in);
+    internal_assert(equal(result, correct))
+        << "Incorrect lossless_cast result:\nlossless_cast("
+        << t << ", "
+        << in
+        << ") gave: "
+        << result
+        << " but expected was: "
+        << correct << "\n";
+}
+
+void lossless_cast_test() {
+    Expr x = Variable::make(Int(32), "x");
+    Type u8 = UInt(8);
+    Type u16 = UInt(16);
+    Type u32 = UInt(32);
+    Type i8 = Int(8);
+    Type i16 = Int(16);
+    Type i32 = Int(32);
+    Type u8x = UInt(8, 4);
+    Type u16x = UInt(16, 4);
+    Type u32x = UInt(32, 4);
+    Expr var_i8 = Variable::make(i8, "x");
+    Expr var_i16 = Variable::make(i16, "x");
+    Expr var_u8 = Variable::make(u8, "x");
+    Expr var_u16 = Variable::make(u16, "x");
+    Expr var_u8x = Variable::make(u8x, "x");
+
+    Expr e = Ramp::make(0, 1, 4);
+    check_lossless_cast(u8x, e, cast(u8x, e));
+
+    // Overflowing ramp
+    e = Ramp::make(make_const(u16, 5), make_const(u16, 32800), 3);
+    check_lossless_cast(u8, e, Expr());
+
+    e = x % 4;
+    check_lossless_cast(UInt(8), e, cast(UInt(8), e));
+
+    e = var_i8 % make_const(i8, -128);
+    check_lossless_cast(UInt(8), e, cast(UInt(8), e));
+
+    e = var_i16 % make_const(i16, -256);
+    check_lossless_cast(UInt(8), e, cast(UInt(8), e));
+
+    e = var_u16 % make_const(u16, 256);
+    check_lossless_cast(UInt(8), e, cast(UInt(8), e));
+
+    e = var_u8 % Variable::make(u8, "y");
+    check_lossless_cast(UInt(8), e, cast(UInt(8), e));
+
+    e = cast(u8, x);
+    check_lossless_cast(i32, e, cast(i32, e));
+
+    e = cast(u8, x);
+    check_lossless_cast(i32, e, cast(i32, e));
+
+    e = cast(i8, var_u16);
+    check_lossless_cast(u16, e, Expr());
+
+    e = cast(i16, var_u16);
+    check_lossless_cast(u16, e, Expr());
+
+    e = cast(u32, var_u8);
+    check_lossless_cast(u16, e, cast(u16, var_u8));
+
+    e = VectorReduce::make(VectorReduce::Add, cast(u16x, var_u8x), 1);
+    check_lossless_cast(u16, e, cast(u16, e));
+
+    e = VectorReduce::make(VectorReduce::Add, cast(u32x, var_u8x), 1);
+    check_lossless_cast(u16, e, VectorReduce::make(VectorReduce::Add, cast(u16x, var_u8x), 1));
+
+    debug(0) << "lossless_cast test passed\n";
+}
+
+}  // namespace Internal
+
 }  // namespace Halide

src/IROperator.h

@@ -154,6 +154,8 @@ Expr const_false(int lanes = 1);
  * Expr. */
 Expr lossless_cast(Type t, Expr e);
 
+void lossless_cast_test();
+
 /** Coerce the two expressions to have the same type, using C-style
  * casting rules. For the purposes of casting, a boolean type is
  * UInt(1). We use the following procedure:

test/internal.cpp

@@ -12,6 +12,7 @@
 #include "IR.h"
 #include "IREquality.h"
 #include "IRMatch.h"
+#include "IROperator.h"
 #include "IRPrinter.h"
 #include "Interval.h"
 #include "ModulusRemainder.h"
@@ -42,6 +43,7 @@ int main(int argc, const char **argv) {
     generator_test();
     propagate_estimate_test();
     uniquify_variable_names_test();
+    lossless_cast_test();
 
     printf("Success!\n");
     return 0;