PGO: Add a run-make test that makes sure that PGO profiling data is used by the compiler during optimizations.

src/test/run-make-fulldeps/pgo-use/Makefile

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+# needs-profiler-support
+
+-include ../tools.mk
+
+# This test makes sure that PGO profiling data leads to cold functions being
+# marked as `cold` and hot functions with `inlinehint`.
+# The test program contains an `if` were actual execution only ever takes the
+# `else` branch. Accordingly, we expect the function that is never called to
+# be marked as cold.
+#
+# The program is compiled with `-Copt-level=s` because this setting disables
+# LLVM's pre-inlining pass (i.e. a pass that does some inlining before it adds
+# the profiling instrumentation). Disabling this pass leads to rather
+# predictable IR which we need for this test to be stable.
+
+COMMON_FLAGS=-Copt-level=s -Ccodegen-units=1
+
+# LLVM doesn't support instrumenting binaries that use SEH:
+# https://bugs.llvm.org/show_bug.cgi?id=41279
+#
+# Things work fine with -Cpanic=abort though.
+ifdef IS_MSVC
+COMMON_FLAGS+= -Cpanic=abort
+endif
+
+all:
+	# Compile the test program with instrumentation
+	$(RUSTC) $(COMMON_FLAGS) -Z pgo-gen="$(TMPDIR)" main.rs
+	# Run it in order to generate some profiling data
+	$(call RUN,main some-argument) || exit 1
+	# Postprocess the profiling data so it can be used by the compiler
+	$(LD_LIB_PATH_ENVVAR)=$(REAL_LD_LIBRARY_PATH) llvm-profdata merge \
+		-o "$(TMPDIR)"/merged.profdata \
+		"$(TMPDIR)"/default_*.profraw
+	# Compile the test program again, making use of the profiling data
+	$(RUSTC) $(COMMON_FLAGS) -Z pgo-use="$(TMPDIR)"/merged.profdata --emit=llvm-ir main.rs
+	# Check that the generate IR contains some things that we expect
+	#
+	# We feed the file into LLVM FileCheck tool *in reverse* so that we see the
+	# line with the function name before the line with the function attributes.
+	# FileCheck only supports checking that something matches on the next line,
+	# but not if something matches on the previous line.
+	tac "$(TMPDIR)"/main.ll | "$(LLVM_FILECHECK)" filecheck-patterns.txt

src/test/run-make-fulldeps/pgo-use/filecheck-patterns.txt

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+# Add a check that the IR contains some expected metadata
+CHECK: !{!"ProfileFormat", !"InstrProf"}
+CHECK: !"ProfileSummary"
+
+# Make sure that the hot function is marked with `inlinehint`
+CHECK: define {{.*}} @hot_function
+CHECK-NEXT: Function Attrs:{{.*}}inlinehint
+
+# Make sure that the cold function is marked with `cold`
+CHECK: define {{.*}} @cold_function
+CHECK-NEXT: Function Attrs:{{.*}}cold

src/test/run-make-fulldeps/pgo-use/main.rs

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+#[no_mangle]
+pub fn cold_function(c: u8) {
+    println!("cold {}", c);
+}
+
+#[no_mangle]
+pub fn hot_function(c: u8) {
+    std::env::set_var(format!("var{}", c), format!("hot {}", c));
+}
+
+fn main() {
+    let arg = std::env::args().skip(1).next().unwrap();
+
+    for i in 0 .. 1000_000 {
+        let some_value = arg.as_bytes()[i % arg.len()];
+        if some_value == b'!' {
+            // This branch is never taken at runtime
+            cold_function(some_value);
+        } else {
+            hot_function(some_value);
+        }
+    }
+}