[Flang] Lower achar intrinsic

The intrinsic returns the character located at the position requested
in the ASCII sequence. The intrinsic is lowered to inline FIR code.

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D122480

Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>

flang/lib/Lower/IntrinsicCall.cpp

@@ -648,6 +648,7 @@ static constexpr bool handleDynamicOptional = true;
 /// should be provided for all the intrinsic arguments for completeness.
 static constexpr IntrinsicHandler handlers[]{
     {"abs", &I::genAbs},
+    {"achar", &I::genChar},
     {"adjustl",
      &I::genAdjustRtCall<fir::runtime::genAdjustL>,
      {{{"string", asAddr}}},

flang/test/Lower/Intrinsics/achar.f90

@@ -0,0 +1,23 @@
+! RUN: bbc -emit-fir %s -o - | fir-opt --canonicalize | FileCheck %s
+! RUN: %flang_fc1 -emit-fir %s -o - | fir-opt --canonicalize | FileCheck %s
+
+! CHECK-LABEL: test1
+! CHECK-SAME: (%[[XREF:.*]]: !fir.ref<i32> {{.*}}, %[[CBOX:.*]]: !fir.boxchar<1> {{.*}})
+! CHECK: %[[C1:.*]] = arith.constant 1 : index
+! CHECK: %[[FALSE:.*]] = arith.constant false
+! CHECK: %[[TEMP:.*]] = fir.alloca !fir.char<1> {adapt.valuebyref}
+! CHECK: %[[C:.*]]:2 = fir.unboxchar %[[CBOX]] : (!fir.boxchar<1>) -> (!fir.ref<!fir.char<1,?>>, index)
+! CHECK: %[[X:.*]] = fir.load %[[XREF]] : !fir.ref<i32>
+! CHECK: %[[X_I8:.*]] = fir.convert %[[X]] : (i32) -> i8
+! CHECK: %[[UNDEF:.*]] = fir.undefined !fir.char<1>
+! CHECK: %[[XCHAR:.*]] = fir.insert_value %[[UNDEF]], %[[X_I8]], [0 : index] : (!fir.char<1>, i8) -> !fir.char<1>
+! CHECK: fir.store %[[XCHAR]] to %[[TEMP]] : !fir.ref<!fir.char<1>>
+! CHECK: %[[C1_I64:.*]] = fir.convert %[[C1]] : (index) -> i64
+! CHECK: %[[C_CVT:.*]] = fir.convert %[[C]]#0 : (!fir.ref<!fir.char<1,?>>) -> !fir.ref<i8>
+! CHECK: %[[TEMP_WITH_XCHAR:.*]] = fir.convert %[[TEMP]] : (!fir.ref<!fir.char<1>>) -> !fir.ref<i8>
+! CHECK: fir.call @llvm.memmove.p0i8.p0i8.i64(%[[C_CVT]], %[[TEMP_WITH_XCHAR]], %[[C1_I64]], %[[FALSE]]) : (!fir.ref<i8>, !fir.ref<i8>, i64, i1) -> ()
+subroutine test1(x, c)
+  integer :: x
+  character :: c
+  c = achar(x)
+end subroutine