[flang] Implement IERRNO intrinsic

Author: JDPailleux

flang/docs/Intrinsics.md

@@ -1064,3 +1064,14 @@ This intrinsic is an alias for `LEN_TRIM`, without the optional KIND argument.
 - **Arguments:** `TIME` - a REAL value into which the elapsed CPU time in
                           seconds is written
 - **RETURN value:** same as TIME argument
+
+### Non-Standard Intrinsics: IERRNO
+
+#### Description
+`IERRNO()` returns the last system error number, as given by the C `errno` variable.
+
+#### Usage and Info
+
+- **Standard:** GNU extension
+- **Class:** function
+- **Syntax:** `RESULT = IERRNO()`

flang/include/flang/Runtime/extensions.h

@@ -69,5 +69,8 @@ std::int64_t FORTRAN_PROCEDURE_NAME(access)(const char *name,
     std::int64_t nameLength, const char *mode, std::int64_t modeLength);
 #endif
 
+// GNU extension function IERRNO()
+int FORTRAN_PROCEDURE_NAME(ierrno)();
+
 } // extern "C"
 #endif // FORTRAN_RUNTIME_EXTENSIONS_H_

flang/runtime/extensions.cpp

@@ -248,5 +248,7 @@ std::int64_t FORTRAN_PROCEDURE_NAME(access)(const char *name,
 }
 #endif
 
+int FORTRAN_PROCEDURE_NAME(ierrno)() { return errno; }
+
 } // namespace Fortran::runtime
 } // extern "C"

flang/test/Lower/Intrinsics/ierrno.f90

@@ -0,0 +1,13 @@
+! RUN: bbc -emit-fir %s -o - | FileCheck --check-prefixes=CHECK %s
+! RUN: %flang_fc1 -emit-fir %s -o - | FileCheck --check-prefixes=CHECK %s
+
+! CHECK-LABEL: func @_QPtest_ierrno(
+subroutine test_ierrno()
+    integer :: i
+    i = ierrno()
+! CHECK: %[[VAL_0:.*]] = fir.alloca i32 {bindc_name = "i", uniq_name = "_QFtest_ierrnoEi"}
+! CHECK: %[[VAL_1:.*]] = fir.declare %[[VAL_0]] {uniq_name = "_QFtest_ierrnoEi"} : (!fir.ref<i32>) -> !fir.ref<i32>
+! CHECK: %[[VAL_2:.*]] = fir.call @_QPierrno() fastmath<contract> : () -> i32
+! CHECK: fir.store %[[VAL_2]] to %[[VAL_1]] : !fir.ref<i32>
+! CHECK: return
+end subroutine test_ierrno