[flang] Legacy extension intrinsic functions IMAG, IZEXT, JZEXT

Support these legacy extension intrinsic functions with unambiguous
semantics in those existing compilers that support them by means
of recognizing them as aliases for standard intrinsics (IMAG) or
with simple rewrites (IZEXT, JZEXT).  Note that ZEXT has different
semantics in different existing compilers, so we will not support it
due to lack of a broad unambiguous precedent.

Differential Revision: https://reviews.llvm.org/D132154

flang/docs/Extensions.md

@@ -235,6 +235,11 @@ end
   respectively.
 * A digit count of d=0 is accepted in Ew.0, Dw.0, and Gw.0 output
   editing if no nonzero scale factor (kP) is in effect.
+* The name `IMAG` is accepted as an alias for the generic intrinsic
+  function `AIMAG`.
+* The legacy extension intrinsic functions `IZEXT` and `JZEXT`
+  are supported; `ZEXT` has different behavior with various older
+  compilers, so it is not supported.
 
 ### Extensions supported when enabled by options
 

flang/lib/Evaluate/fold-integer.cpp

@@ -762,6 +762,17 @@ Expr<Type<TypeCategory::Integer, KIND>> FoldIntrinsicFunction(
                 return i.ISHFTC(countVal);
               }));
     }
+  } else if (name == "izext" || name == "jzext") {
+    if (args.size() == 1) {
+      if (auto *expr{UnwrapExpr<Expr<SomeInteger>>(args[0])}) {
+        // Rewrite to IAND(INT(n,k),255_k) for k=KIND(T)
+        intrinsic->name = "iand";
+        auto converted{ConvertToType<T>(std::move(*expr))};
+        *expr = Fold(context, Expr<SomeInteger>{std::move(converted)});
+        args.emplace_back(AsGenericExpr(Expr<T>{Scalar<T>{255}}));
+        return FoldIntrinsicFunction(context, std::move(funcRef));
+      }
+    }
   } else if (name == "lbound") {
     return LBOUND(context, std::move(funcRef));
   } else if (name == "leadz" || name == "trailz" || name == "poppar" ||

flang/lib/Evaluate/intrinsics.cpp

@@ -507,6 +507,8 @@ static const IntrinsicInterface genericIntrinsicFunction[]{
         DefaultLogical, Rank::elemental, IntrinsicClass::inquiryFunction},
     {"is_iostat_end", {{"i", AnyInt}}, DefaultLogical},
     {"is_iostat_eor", {{"i", AnyInt}}, DefaultLogical},
+    {"izext", {{"i", AnyInt}}, TypePattern{IntType, KindCode::exactKind, 2}},
+    {"jzext", {{"i", AnyInt}}, DefaultInt},
     {"kind", {{"x", AnyIntrinsic}}, DefaultInt, Rank::elemental,
         IntrinsicClass::inquiryFunction},
     {"lbound",
@@ -844,7 +846,7 @@ static const IntrinsicInterface genericIntrinsicFunction[]{
 // TODO: Coarray intrinsic functions
 //   LCOBOUND, UCOBOUND, IMAGE_INDEX, COSHAPE
 // TODO: Non-standard intrinsic functions
-//  LSHIFT, RSHIFT, SHIFT, ZEXT, IZEXT,
+//  LSHIFT, RSHIFT, SHIFT,
 //  COMPL, EQV, NEQV, INT8, JINT, JNINT, KNINT,
 //  QCMPLX, QEXT, QFLOAT, QREAL, DNUM,
 //  INUM, JNUM, KNUM, QNUM, RNUM, RAN, RANF, ILEN,
@@ -860,6 +862,7 @@ static const IntrinsicInterface genericIntrinsicFunction[]{
 // compatibility and builtins.
 static const std::pair<const char *, const char *> genericAlias[]{
     {"and", "iand"},
+    {"imag", "aimag"},
     {"or", "ior"},
     {"xor", "ieor"},
     {"__builtin_ieee_selected_real_kind", "selected_real_kind"},

flang/test/Evaluate/fold-jzext.f90

@@ -0,0 +1,10 @@
+! RUN: %python %S/test_folding.py %s %flang_fc1
+! Tests folding of IZEXT() & JZEXT()
+module m
+  logical, parameter :: test_1 = kind(izext(-1_1)) == 2
+  logical, parameter :: test_2 = izext(-1_1) == 255_2
+  logical, parameter :: test_3 = kind(jzext(-1_1)) == 4
+  logical, parameter :: test_4 = jzext(-1_1) == 255_4
+  logical, parameter :: test_5 = kind(jzext(-1_2)) == 4
+  logical, parameter :: test_6 = jzext(-1_2) == 255_4
+end module