feat: add --fast-build dev-iteration build mode (prototype)

New 'Fast' build type for fast edit-rebuild-run iteration (e.g. GPU print
debugging). It matches none of the Release-only (IPO, -march=native) or
Debug/RelDebug-only (MFC_DEBUG, -gpu=debug) conditional blocks, so it inherits
none of them; adds a light -O1. On NVHPC GPU builds it autodetects the node's
single compute capability (nvidia-smi) and overrides the multi-arch MFC_CUDA_CC,
with MFC_FAST_ARCH as a login-node escape hatch.

Measured (NVHPC 24.5, RTX 6000 cc75, generic simulation, 8 cores):
  clean build  641s (Release fat 5-arch) -> 170s  (3.8x)
  hot-module   385s (Release fat 5-arch) ->  79s  (4.9x)
Verified: builds with no IPO/MFC_DEBUG, runs a 1D case on GPU to exit 0.

Adds fast_build to MFCConfig (auto --fast-build/--no-fast-build, own slug);
bumps lock version to 9 for the new config field.

Author: sbryngelson

CMakeLists.txt

@@ -40,9 +40,9 @@ if (MFC_ALL)
 endif()
 
 # Validate CMAKE_BUILD_TYPE to catch typos (CMake is case-sensitive).
-set(_VALID_BUILD_TYPES "Debug" "Release" "RelDebug" "")
+set(_VALID_BUILD_TYPES "Debug" "Release" "RelDebug" "Fast" "")
 if (NOT CMAKE_BUILD_TYPE IN_LIST _VALID_BUILD_TYPES)
-    message(FATAL_ERROR "Unknown CMAKE_BUILD_TYPE '${CMAKE_BUILD_TYPE}'. Valid: Debug, RelDebug, Release")
+    message(FATAL_ERROR "Unknown CMAKE_BUILD_TYPE '${CMAKE_BUILD_TYPE}'. Valid: Debug, RelDebug, Release, Fast")
 endif()
 
 # RelDebug: a lighter debug mode for CI. Compiler-specific blocks below add the
@@ -51,6 +51,13 @@ set(CMAKE_C_FLAGS_RELDEBUG       "-g" CACHE STRING "")
 set(CMAKE_CXX_FLAGS_RELDEBUG     "-g" CACHE STRING "")
 set(CMAKE_Fortran_FLAGS_RELDEBUG "-g" CACHE STRING "")
 
+# Fast: fast-iteration dev builds (e.g. GPU print-debugging). Deliberately matches
+# none of the Release-only (IPO, -march=native) or Debug/RelDebug-only (MFC_DEBUG,
+# -gpu=debug) conditional blocks below, so it inherits none of them - just a light -O1.
+set(CMAKE_C_FLAGS_FAST       "-O1" CACHE STRING "")
+set(CMAKE_CXX_FLAGS_FAST     "-O1" CACHE STRING "")
+set(CMAKE_Fortran_FLAGS_FAST "-O1" CACHE STRING "")
+
 if (MFC_SINGLE_PRECISION)
     add_compile_definitions(MFC_SINGLE_PRECISION)
 else()
@@ -330,6 +337,14 @@ if (CMAKE_BUILD_TYPE STREQUAL "Debug" OR CMAKE_BUILD_TYPE STREQUAL "RelDebug")
     add_compile_definitions(MFC_DEBUG)
 endif()
 
+# Fast: light optimization for dev iteration. Like Debug/RelDebug, the real opt
+# flag is injected here (the CMAKE_*_FLAGS_FAST cache vars are placeholders).
+# -O1 keeps compile time low while giving acceptable runtime; no MFC_DEBUG, so
+# device routines stay free of host-only debug aborts and compile without IPO.
+if (CMAKE_BUILD_TYPE STREQUAL "Fast")
+    add_compile_options($<$<COMPILE_LANGUAGE:Fortran>:-O1>)
+endif()
+
 
 
 # HANDLE_SOURCES: Given a target (herein <target>):

toolchain/mfc/build.py

@@ -27,6 +27,53 @@
 _MAKE_PROGRESS_RE = re.compile(r"^\[\s*(\d+)%\]\s+(.*)$")
 
 
+def _cmake_build_type() -> str:
+    """Map the CLI build-mode flags to a CMAKE_BUILD_TYPE string."""
+    if ARG("debug"):
+        return "Debug"
+    if ARG("reldebug"):
+        return "RelDebug"
+    if ARG("fast_build", False):
+        return "Fast"
+    return "Release"
+
+
+def _apply_fast_build_gpu_arch() -> None:
+    """Under --fast-build on an NVHPC GPU build, restrict device codegen to a
+    single compute capability (the node's GPU), overriding the multi-arch
+    MFC_CUDA_CC that the module files set. CMake reads $ENV{MFC_CUDA_CC}.
+
+    Cray/AMD GPU builds don't use MFC_CUDA_CC (they are already single-arch via
+    craype-accel/--offload-arch), so this only acts when MFC_CUDA_CC is set.
+    Hard-errors if no GPU is detectable and no explicit arch is provided."""
+    if not ARG("fast_build", False) or ARG("gpu") == gpuConfigOptions.NONE.value:
+        return
+    if not os.environ.get("MFC_CUDA_CC"):  # not an NVHPC node; nothing to do
+        return
+
+    override = os.environ.get("MFC_FAST_ARCH")  # escape hatch for login nodes
+    if override:
+        os.environ["MFC_CUDA_CC"] = override
+        return
+
+    try:
+        result = subprocess.run(
+            ["nvidia-smi", "--query-gpu=compute_cap", "--format=csv,noheader"],
+            capture_output=True,
+            text=True,
+            timeout=10,
+            check=False,
+        )
+        caps = [ln.strip().replace(".", "") for ln in result.stdout.splitlines() if ln.strip()]
+    except (OSError, subprocess.SubprocessError):
+        caps = []
+
+    if not caps:
+        raise MFCException("--fast-build: could not detect a local GPU compute capability " "(no GPU visible via nvidia-smi). Run on a GPU node, or set " "MFC_FAST_ARCH=<cc> (e.g. MFC_FAST_ARCH=90).")
+
+    os.environ["MFC_CUDA_CC"] = caps[0]
+
+
 def _run_build_with_progress(command: typing.List[str], target_name: str, streaming: bool = False) -> subprocess.CompletedProcess:
     """
     Run a build command with a progress bar that parses ninja output.
@@ -367,6 +414,10 @@ def is_buildable(self) -> bool:
     def configure(self, case: Case):
         if ARG("debug") and ARG("reldebug"):
             raise MFCException("--debug and --reldebug are mutually exclusive.")
+        if ARG("fast_build", False) and (ARG("debug") or ARG("reldebug")):
+            raise MFCException("--fast-build is mutually exclusive with --debug/--reldebug.")
+
+        _apply_fast_build_gpu_arch()
 
         build_dirpath = self.get_staging_dirpath(case)
         cmake_dirpath = self.get_cmake_dirpath()
@@ -386,9 +437,9 @@ def configure(self, case: Case):
             # build the configured targets. This is mostly useful for debugging.
             # See: https://cmake.org/cmake/help/latest/variable/CMAKE_EXPORT_COMPILE_COMMANDS.html.
             "-DCMAKE_EXPORT_COMPILE_COMMANDS=ON",
-            # Set build type (Debug, RelDebug, or Release).
+            # Set build type (Debug, RelDebug, Fast, or Release).
             # See: https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html
-            f"-DCMAKE_BUILD_TYPE={'Debug' if ARG('debug') else 'RelDebug' if ARG('reldebug') else 'Release'}",
+            f"-DCMAKE_BUILD_TYPE={_cmake_build_type()}",
             # Used by FIND_PACKAGE (/FindXXX) to search for packages, with the
             # second highest level of priority, still letting users manually
             # specify <PackageName>_ROOT, which has precedence over CMAKE_PREFIX_PATH.
@@ -468,7 +519,7 @@ def build(self, case: input.MFCInputFile):
             "--parallel",
             ARG("jobs"),
             "--config",
-            "Debug" if ARG("debug") else "RelDebug" if ARG("reldebug") else "Release",
+            _cmake_build_type(),
         ]
 
         verbosity = ARG("verbose")

toolchain/mfc/lock.py

@@ -5,7 +5,7 @@
 from .printer import cons
 from .state import MFCConfig
 
-MFC_LOCK_CURRENT_VERSION: int = 8
+MFC_LOCK_CURRENT_VERSION: int = 9
 
 
 @dataclasses.dataclass

toolchain/mfc/state.py

@@ -16,6 +16,7 @@ class MFCConfig:
     gpu: str = gpuConfigOptions.NONE.value
     debug: bool = False
     reldebug: bool = False
+    fast_build: bool = False
     gcov: bool = False
     unified: bool = False
     single: bool = False