[flang] Register and lower SECNDS (stubbed implementation) (#151878)

This patch registers and lowers the GNU extension intrinsic `SECNDS` in
Flang.

- Registration and lowering are wired through the intrinsic
infrastructure.
- genSecnds() currently emits a TODO fatal error, marking it as
unimplemented.
- Includes an XFAIL test to exercise the lowering path and reflect
current status.

Fixes https://github.com/llvm/llvm-project/issues/58728

---

CC @eugeneepshteyn @klausler

---------

Co-authored-by: Eugene Epshteyn <eepshteyn@nvidia.com>

flang/docs/Intrinsics.md

@@ -1123,6 +1123,33 @@ program rename_proc
 end program rename_proc
 ```
 
+### Non-Standard Intrinsics: SECNDS
+#### Description
+`SECNDS(refTime)` returns the number of seconds since midnight minus a user-supplied reference time `refTime`. If the difference is negative (i.e., the current time is past midnight and refTime was from the previous day), the result wraps around midnight to yield a positive value.
+
+#### Usage and Info
+- **Standard:**  GNU extension
+- **Class:**     function
+- **Syntax:**    result = `SECNDS(refTime)`
+- **Arguments:** 
+ 
+| ARGUMENT  | INTENT |      TYPE     |          KIND           |           Description                    |
+|-----------|--------|---------------|-------------------------|------------------------------------------|
+| `refTime` | `IN`   | `REAL, scalar`| REAL(KIND=4), required  | Reference time in seconds since midnight |
+
+- **Return Value:** REAL(KIND=4), scalar — seconds elapsed since `refTime`.
+- **Purity:**       Impure. SECNDS references the system clock and may not be invoked from a PURE procedure.
+
+#### Example
+```Fortran
+PROGRAM example_secnds
+  REAL :: refTime, elapsed
+  refTime = SECNDS(0.0)
+  elapsed = SECNDS(refTime)
+  PRINT *, "Elapsed seconds:", elapsed
+END PROGRAM example_secnds
+```
+
 ### Non-standard Intrinsics: SECOND
 This intrinsic is an alias for `CPU_TIME`: supporting both a subroutine and a
 function form.

flang/include/flang/Optimizer/Builder/IntrinsicCall.h

@@ -407,6 +407,8 @@ struct IntrinsicLibrary {
                                    llvm::ArrayRef<fir::ExtendedValue>);
   mlir::Value genScale(mlir::Type, llvm::ArrayRef<mlir::Value>);
   fir::ExtendedValue genScan(mlir::Type, llvm::ArrayRef<fir::ExtendedValue>);
+  fir::ExtendedValue genSecnds(mlir::Type resultType,
+                               llvm::ArrayRef<fir::ExtendedValue> args);
   fir::ExtendedValue genSecond(std::optional<mlir::Type>,
                                mlir::ArrayRef<fir::ExtendedValue>);
   fir::ExtendedValue genSelectedCharKind(mlir::Type,

flang/include/flang/Optimizer/Builder/Runtime/Intrinsics.h

@@ -70,6 +70,9 @@ void genRandomSeed(fir::FirOpBuilder &, mlir::Location, mlir::Value size,
 void genRename(fir::FirOpBuilder &builder, mlir::Location loc,
                mlir::Value path1, mlir::Value path2, mlir::Value status);
 
+mlir::Value genSecnds(fir::FirOpBuilder &builder, mlir::Location loc,
+                      mlir::Value refTime);
+
 /// generate time runtime call
 mlir::Value genTime(fir::FirOpBuilder &builder, mlir::Location loc);
 

flang/lib/Evaluate/intrinsics.cpp

@@ -921,6 +921,10 @@ static const IntrinsicInterface genericIntrinsicFunction[]{
             {"back", AnyLogical, Rank::elemental, Optionality::optional},
             DefaultingKIND},
         KINDInt},
+    {"secnds",
+        {{"refTime", TypePattern{RealType, KindCode::exactKind, 4},
+            Rank::scalar}},
+        TypePattern{RealType, KindCode::exactKind, 4}, Rank::scalar},
     {"second", {}, DefaultReal, Rank::scalar},
     {"selected_char_kind", {{"name", DefaultChar, Rank::scalar}}, DefaultInt,
         Rank::scalar, IntrinsicClass::transformationalFunction},

flang/lib/Optimizer/Builder/IntrinsicCall.cpp

@@ -869,6 +869,10 @@ static constexpr IntrinsicHandler handlers[]{
        {"back", asValue, handleDynamicOptional},
        {"kind", asValue}}},
      /*isElemental=*/true},
+    {"secnds",
+     &I::genSecnds,
+     {{{"refTime", asAddr}}},
+     /*isElemental=*/false},
     {"second",
      &I::genSecond,
      {{{"time", asAddr}}},
@@ -7864,6 +7868,22 @@ IntrinsicLibrary::genScan(mlir::Type resultType,
   return readAndAddCleanUp(resultMutableBox, resultType, "SCAN");
 }
 
+// SECNDS
+fir::ExtendedValue
+IntrinsicLibrary::genSecnds(mlir::Type resultType,
+                            llvm::ArrayRef<fir::ExtendedValue> args) {
+  assert(args.size() == 1 && "SECNDS expects one argument");
+
+  mlir::Value refTime = fir::getBase(args[0]);
+
+  if (!refTime)
+    fir::emitFatalError(loc, "expected REFERENCE TIME parameter");
+
+  mlir::Value result = fir::runtime::genSecnds(builder, loc, refTime);
+
+  return builder.createConvert(loc, resultType, result);
+}
+
 // SECOND
 fir::ExtendedValue
 IntrinsicLibrary::genSecond(std::optional<mlir::Type> resultType,

flang/lib/Optimizer/Builder/Runtime/Intrinsics.cpp

@@ -276,6 +276,23 @@ void fir::runtime::genRename(fir::FirOpBuilder &builder, mlir::Location loc,
   fir::CallOp::create(builder, loc, runtimeFunc, args);
 }
 
+mlir::Value fir::runtime::genSecnds(fir::FirOpBuilder &builder,
+                                    mlir::Location loc, mlir::Value refTime) {
+  auto runtimeFunc =
+      fir::runtime::getRuntimeFunc<mkRTKey(Secnds)>(loc, builder);
+
+  mlir::FunctionType runtimeFuncTy = runtimeFunc.getFunctionType();
+
+  mlir::Value sourceFile = fir::factory::locationToFilename(builder, loc);
+  mlir::Value sourceLine =
+      fir::factory::locationToLineNo(builder, loc, runtimeFuncTy.getInput(2));
+
+  llvm::SmallVector<mlir::Value> args = {refTime, sourceFile, sourceLine};
+  args = fir::runtime::createArguments(builder, loc, runtimeFuncTy, args);
+
+  return fir::CallOp::create(builder, loc, runtimeFunc, args).getResult(0);
+}
+
 /// generate runtime call to time intrinsic
 mlir::Value fir::runtime::genTime(fir::FirOpBuilder &builder,
                                   mlir::Location loc) {

flang/test/Lower/Intrinsics/secnds.f90

@@ -0,0 +1,23 @@
+! RUN: bbc -emit-fir -hlfir=false %s -o - | FileCheck %s
+
+! CHECK-LABEL: func.func @_QPuse_secnds(
+! CHECK-SAME: %arg0: !fir.ref<f32>
+function use_secnds(refTime) result(elapsed)
+  real :: refTime, elapsed
+  elapsed = secnds(refTime)
+end function
+
+! File/line operands (don’t match the actual path/number)
+! CHECK: %[[STRADDR:.*]] = fir.address_of(
+! CHECK: %[[LINE:.*]] = arith.constant {{.*}} : i32
+! CHECK: %[[FNAME8:.*]] = fir.convert %[[STRADDR]] : (!fir.ref<!fir.char<1,{{.*}}>>) -> !fir.ref<i8>
+
+! Important: pass refTime by address and return a value f32
+! CHECK: %[[CALL:.*]] = fir.call @{{.*}}Secnds(%arg0, %[[FNAME8]], %[[LINE]]) {{.*}} : (!fir.ref<f32>, !fir.ref<i8>, i32) -> f32
+
+! Guard against illegal value ->ref conversion of result
+! CHECK-NOT: fir.convert {{.*}} : (f32) -> !fir.ref<f32>
+
+! Function returns an f32 value
+! CHECK: return {{.*}} : f32
+