gh-xxxx: overhaul build rules for optimized binaries

This commit overhauls the make-based build system's rules for
building optimized binaries. Along the way it fixes a myriad of
bugs and shortcomings with the prior approach.

The old way of producing optimized binaries had various limitations:

* `make [all]` would do work when PGO was enabled because the phony
  `profile-opt` rule was non-empty. This prevented no-op PGO builds
   from working at all. This meant workflows like `make; make install`
   either incurred extra work or failed due to race conditions.
* Same thing for BOLT, as its `bolt-opt` rule was also non-empty
  and always ran during `make [all]`.
* BOLT could not be run multiple times without a full rebuild because
  `llvm-bolt` can't instrument binaries that have already received
  BOLT optimizations.
* It was difficult to run BOLT on its own because of how various make
  targets and their dependencies were structured.
* I found the old way that configure and make communicated the default
  targets to be confusing and hard to understand.

There are essentially 2 major changes going on in this commit:

1. A rework of the high-level make targets for performing a build and
   how they are defined.
2. A rework of all the make logic related to profile-based optimization
   (read: PGO and BOLT).

Build Target Rework
======================

Before, we essentially had `build_all`, `profile-opt`, `bolt-opt` and
`build_wasm` as our 3 targets for performing a build. `all` would alias
to one of these, as appropriate.

And there was another definition for which _simple_ make target to
evaluate for non-optimized builds. This was likely `build_all` or
`all`.

In the rework, we introduce 2 new high-level targets:

* `build-plain` - Perform a build without optimizations.
* `build-optimized` - Perform a build with optimizations.

`build-plain` is aliased to `build_all` in all configurations except
WASM, where it is `build_wasm`.

`build-optimized` by default is aliased to a target that prints an error
message when optimizations aren't enabled. If PGO or BOLT are enabled,
it is aliased to their respective target.

`build-optimized` is the logical successor to `profile-opt`.

I felt it best to delete `profile-opt` completely, as the new `build-*`
high-level targets feel more friendly to use. But if people lament its
loss, we can add a `profile-opt: build-optimized` to achieve almost the
same result.

Profiled-Based Optimization Rework
==================================

Most of the make logic related to profile-based optimization (read: PGO
and BOLT) has been touched in this change.

A major issue with the old way of doing things was we used phony,
always-executed make rules. This is a bad practice in make because it
undermines no-op builds.

Another issue is that the separation between the rules and what order
they ran in wasn't always clear. Both PGO and BOLT consist of the same
4 phase solution: instrument, run, analyze, and apply. However, these
steps weren't clearly expressed in the make logic. This is especially
true for BOLT, which only had 1 make rule.

Another issue with BOLT is that it was really easy to get things into
a bad state. e.g. if you applied BOLT to `pythonX.Y` you could not
run BOLT again unless you rebuilt `pythonX.Y` from source.

In the new world, we have separate `profile-<tool>-<stage>-stamp`
rules defining the 4 distinct `instrument`, `run`, `analyze`, and
`apply` stages for both PGO and BOLT. Each of these stages is tracked
by a _stamp_ semaphore file so progress can be captured. This should
all be pretty straightforward.

There is some minimal complexity here to handle BOLT's optional
dependency on PGO, as BOLT either depends on `build_all` or
`profile-pgo-apply-stamp`.

As part of the refactor to BOLT we also preserve the original input
binary before BOLT is applied. This original file is restored if
BOLT runs again. This greatly simplifies repeated BOLT invocations,
as make doesn't perform needless work. However, this is all best
effort, as it is possible for some make target evaluations to still
get things in a bad state.

Other Remarks
=============

If this change perturbs any bugs, they are likely around cleaning
behavior. The cleaning rules are a bit complicated and not clearly
documented. And I'm unsure which targets CPython developers often
iterate on. It is highly possible that state cleanup of PGO and/or
BOLT files isn't as robust as it needs to be.

I explicitly deleted some calls to PGO cleanup because those calls
prevented no-op `make [all]` from working. It is certainly possible
something somewhere (release automation?) relied on these files being
deleted when they no longer are. We still have targets to purge profile
files and it should be trivial to add these to appropriate make rules.

Author: indygreg

.gitignore

@@ -122,8 +122,7 @@ Tools/unicode/data/
 # hendrikmuhs/ccache-action@v1
 /.ccache
 /platform
-/profile-clean-stamp
-/profile-run-stamp
+/profile-*-stamp
 /Python/deepfreeze/*.c
 /pybuilddir.txt
 /pyconfig.h

Makefile.pre.in

@@ -577,12 +577,26 @@ LIBEXPAT_HEADERS= \
 #########################################################################
 # Rules
 
-# Default target
-all:		@DEF_MAKE_ALL_RULE@
+# Default target.
+# Likely either `build-plain` or `build-optimized`.
+all: @MAKE_TARGET_ALL@
 # First target in Makefile is implicit default. So .PHONY needs to come after
 # all.
 .PHONY: all
 
+# Build without any optimizations or instrumented binaries.
+.PHONY: build-plain
+build-plain: @MAKE_TARGET_BUILD_PLAIN@
+
+# Build with optimizations (PGO, BOLT, etc).
+.PHONY: build-optimized
+build-optimized: @MAKE_TARGET_BUILD_OPTIMIZED@
+
+.PHONY: build-optimized-not-enabled
+build-optimized-not-enabled:
+	@echo "build-optimized requires --enable-optimizations in configure; aborting"
+	@exit 1
+
 .PHONY: build_all
 build_all:	check-clean-src $(BUILDPYTHON) platform sharedmods \
 		gdbhooks Programs/_testembed scripts checksharedmods rundsymutil
@@ -603,64 +617,145 @@ check-clean-src:
 		exit 1; \
 	fi
 
-# Profile generation build must start from a clean tree.
+# Profile-based optimization.
+#
+# PGO and BOLT profile-based optimization is supported. For each optimization,
+# roughly the following steps are done:
+#
+# 1. "Instrument" binaries with run-time data collection (e.g. build or modify
+#    a variant of the binary.)
+# 2. "Run" instrumented binaries (via subset of test suite) to collect data.
+# 3. "Analyze" / collect / merge data files from previous step.
+# 4. "Apply" collected data from above. (e.g. rebuild or modify a binary).
+#
+# 0, 1, or multiple profile based optimizations can be enabled.
+#
+# We track the progress of profile-based optimization using various "stamp"
+# files. An empty stamp file tracks the stage of optimization we're in.
+# Each *-stamp rule that follows is defined in execution / dependency order.
+
+# Remove files produced by or used for tracking profile-guided optimization.
+.PHONY: profile-remove
+profile-remove: clean-bolt
+	find . -name '*.gc??' -exec rm -f {} ';'
+	find . -name '*.profclang?' -exec rm -f {} ';'
+	find . -name '*.dyn' -exec rm -f {} ';'
+	rm -f $(COVERAGE_INFO)
+	rm -rf $(COVERAGE_REPORT)
+	# Remove all progress tracking stamps to ensure a clean slate.
+	rm -f profile-*-stamp
+
+# Profile-based optimization requires a fresh build environment.
 profile-clean-stamp:
-	$(MAKE) clean
+	$(MAKE) clean profile-remove
 	touch $@
 
-# Compile with profile generation enabled.
-profile-gen-stamp: profile-clean-stamp
+# Build with PGO instrumentation enabled.
+profile-pgo-instrument-stamp: profile-clean-stamp
 	@if [ $(LLVM_PROF_ERR) = yes ]; then \
 		echo "Error: Cannot perform PGO build because llvm-profdata was not found in PATH" ;\
 		echo "Please add it to PATH and run ./configure again" ;\
 		exit 1;\
 	fi
 	@echo "Building with support for profile generation:"
-	$(MAKE) @DEF_MAKE_RULE@ CFLAGS_NODIST="$(CFLAGS_NODIST) $(PGO_PROF_GEN_FLAG)" LDFLAGS_NODIST="$(LDFLAGS_NODIST) $(PGO_PROF_GEN_FLAG)" LIBS="$(LIBS)"
+	$(MAKE) @MAKE_TARGET_BUILD_PLAIN@ CFLAGS_NODIST="$(CFLAGS_NODIST) $(PGO_PROF_GEN_FLAG)" LDFLAGS_NODIST="$(LDFLAGS_NODIST) $(PGO_PROF_GEN_FLAG)" LIBS="$(LIBS)"
 	touch $@
 
-# Run task with profile generation build to create profile information.
-profile-run-stamp:
+# Run PGO instrumented binaries and collect profile data.
+profile-pgo-run-stamp: profile-pgo-instrument-stamp
 	@echo "Running code to generate profile data (this can take a while):"
-	# First, we need to create a clean build with profile generation
-	# enabled.
-	$(MAKE) profile-gen-stamp
-	# Next, run the profile task to generate the profile information.
 	@ # FIXME: can't run for a cross build
 	$(LLVM_PROF_FILE) $(RUNSHARED) ./$(BUILDPYTHON) $(PROFILE_TASK) || true
+	touch $@
+
+# Collect data files produced by running PGO instrumented binaries.
+profile-pgo-analyze-stamp: profile-pgo-run-stamp
 	$(LLVM_PROF_MERGER)
 	# Remove profile generation binary since we are done with it.
 	$(MAKE) clean-retain-profile
-	# This is an expensive target to build and it does not have proper
-	# makefile dependency information.  So, we create a "stamp" file
-	# to record its completion and avoid re-running it.
 	touch $@
 
-# Compile Python binary with profile guided optimization.
-# To force re-running of the profile task, remove the profile-run-stamp file.
-.PHONY: profile-opt
-profile-opt: profile-run-stamp
+# Use collected PGO data to influence rebuild of binaries.
+profile-pgo-apply-stamp: profile-pgo-analyze-stamp
 	@echo "Rebuilding with profile guided optimizations:"
-	-rm -f profile-clean-stamp
-	$(MAKE) @DEF_MAKE_RULE@ CFLAGS_NODIST="$(CFLAGS_NODIST) $(PGO_PROF_USE_FLAG)" LDFLAGS_NODIST="$(LDFLAGS_NODIST)"
+	# Need to purge PGO instrumented build to force a rebuild.
+	$(MAKE) clean-retain-profile
+	$(MAKE) @MAKE_TARGET_BUILD_PLAIN@ CFLAGS_NODIST="$(CFLAGS_NODIST) $(PGO_PROF_USE_FLAG)" LDFLAGS_NODIST="$(LDFLAGS_NODIST)"
+	touch $@
+
+# BOLT supports instrumenting and applying changes to standalone binaries
+# without having to recompile.
+#
+# BOLT can run independently or in addition to PGO. If running with PGO,
+# it always runs after PGO. Care needs to be taken to preserve PGO state
+# when running BOLT so make doesn't re-apply PGO.
+#
+# BOLT also can't instrument binaries that have already had BOLT applied
+# to them. So we make an attempt to preserve and re-use the pristine
+# pre-BOLT binaries so developers can iterate on just BOLT optimization
+# passes.
+
+# List of binaries that BOLT runs on.
+BOLT_BINARIES = $(BUILDPYTHON)
+
+# Remove traces of bolt.
+.PHONY: clean-bolt
+clean-bolt:
+	# Instrumented binaries.
+	find . -name '*.bolt_inst' -exec rm -f {} ';'
+	# The data files they produce.
+	find . -name '*.fdata' -exec rm -f {} ';'
+	# Copied of binaries before BOLT application.
+	find . -name '*.prebolt' -exec rm -f {} ';'
+
+# BOLTs dependencies are a bit wonky.
+#
+# If PGO is enabled, we can take a native rule dependency on a stamp file.
+# If PGO isn't enabled, we don't have a stamp to key off of and the phony
+# target (e.g. build_all) will always force rebuilds. So we call out to
+# make externally to sidestep the dependency.
+#
+# We can simplify this hack if we ever get stamp files for plain builds.
+profile-bolt-prebuild-stamp: @MAKE_BOLT_NATIVE_DEPENDENCY@
+	if [ -n "@MAKE_BOLT_MAKE_DEPENDENCY@" ]; then \
+	    $(MAKE) @MAKE_BOLT_MAKE_DEPENDENCY@; \
+	fi
+	touch $@
 
-.PHONY: bolt-opt
-bolt-opt: @PREBOLT_RULE@
-	rm -f *.fdata
+profile-bolt-instrument-stamp: profile-bolt-prebuild-stamp
+	for bin in $(BOLT_BINARIES); do \
+	    if [ -e "$${bin}.prebolt" ]; then \
+	        echo "Restoring pre-BOLT binary $${bin}.prebolt"; \
+	        mv "$${bin}.prebolt" "$${bin}"; \
+	    fi \
+	done
+	# Ensure prior BOLT state is purged.
+	$(MAKE) clean-bolt
 	@LLVM_BOLT@ ./$(BUILDPYTHON) -instrument -instrumentation-file-append-pid -instrumentation-file=$(abspath $(BUILDPYTHON).bolt) -o $(BUILDPYTHON).bolt_inst
+	touch $@
+
+profile-bolt-run-stamp: profile-bolt-instrument-stamp
 	./$(BUILDPYTHON).bolt_inst $(PROFILE_TASK) || true
+	touch $@
+
+profile-bolt-analyze-stamp: profile-bolt-run-stamp
 	@MERGE_FDATA@ $(BUILDPYTHON).*.fdata > $(BUILDPYTHON).fdata
+	touch $@
+
+profile-bolt-apply-stamp: profile-bolt-analyze-stamp
 	@LLVM_BOLT@ ./$(BUILDPYTHON) -o $(BUILDPYTHON).bolt -data=$(BUILDPYTHON).fdata -update-debug-sections -reorder-blocks=ext-tsp -reorder-functions=hfsort+ -split-functions=3 -icf=1 -inline-all -split-eh -reorder-functions-use-hot-size -peepholes=all -jump-tables=aggressive -inline-ap -indirect-call-promotion=all -dyno-stats -use-gnu-stack -frame-opt=hot
-	rm -f *.fdata
-	rm -f $(BUILDPYTHON).bolt_inst
+	mv $(BUILDPYTHON) $(BUILDPYTHON).prebolt
 	mv $(BUILDPYTHON).bolt $(BUILDPYTHON)
+	touch $@
+
+# End of profile-based optimization rules.
 
 # Compile and run with gcov
 .PHONY: coverage
 coverage:
 	@echo "Building with support for coverage checking:"
 	$(MAKE) clean
-	$(MAKE) @DEF_MAKE_RULE@ CFLAGS="$(CFLAGS) -O0 -pg --coverage" LDFLAGS="$(LDFLAGS) --coverage"
+	$(MAKE) @MAKE_SIMPLE_BUILD_TARGET@ CFLAGS="$(CFLAGS) -O0 -pg --coverage" LDFLAGS="$(LDFLAGS) --coverage"
 
 .PHONY: coverage-lcov
 coverage-lcov:
@@ -2527,23 +2622,9 @@ clean-retain-profile: pycremoval
 	-rm -f Python/frozen_modules/MANIFEST
 	-find build -type f -a ! -name '*.gc??' -exec rm -f {} ';'
 	-rm -f Include/pydtrace_probes.h
-	-rm -f profile-gen-stamp
-
-.PHONY: profile-removal
-profile-removal:
-	find . -name '*.gc??' -exec rm -f {} ';'
-	find . -name '*.profclang?' -exec rm -f {} ';'
-	find . -name '*.dyn' -exec rm -f {} ';'
-	rm -f $(COVERAGE_INFO)
-	rm -rf $(COVERAGE_REPORT)
-	rm -f profile-run-stamp
 
 .PHONY: clean
 clean: clean-retain-profile
-	@if test @DEF_MAKE_ALL_RULE@ = profile-opt; then \
-		rm -f profile-gen-stamp profile-clean-stamp; \
-		$(MAKE) profile-removal; \
-	fi
 
 .PHONY: clobber
 clobber: clean