Bug 1697215 - Disable value profiling in Rust PGO until we move to to rust 1.52. r=firefox-build-system-reviewers,glandium

Credit goes to dmajor.
Rust gets LLVM 12 - rust-lang/rust#81451.
The PGO format changed between 11 and 12, and builds will have poor optimization if we mix versions in xLTO.
The above PR has landed on Rust nightly 1.52.
Until we move to 1.52 nightly we should have this landed.

In the meantime, for unblocking try builds, we can disable value-PGO in Rust.

Differential Revision: https://phabricator.services.mozilla.com/D113327

Author: abpostelnicu

build/moz.configure/rust.configure

@@ -120,6 +120,12 @@ set_config(
 )
 
 
+set_config(
+    "RUSTC_LLVM_VERSION",
+    depends(rustc_info)(lambda info: str(info.llvm_version) if info else None),
+)
+
+
 @depends_if(cargo)
 @checking("cargo version", lambda info: info.version)
 @imports("re")

config/makefiles/rust.mk

@@ -239,6 +239,11 @@ ifdef MOZ_PROFILE_GENERATE
 # because -C panic=unwind (the compiler default) is not compatible with -C profile-generate
 # (https://github.com/rust-lang/rust/issues/61002).
 rust_pgo_flags := -C panic=abort -C profile-generate=$(topobjdir)
+ifeq (1,$(words $(filter 5.% 6.% 7.% 8.% 9.% 10.% 11.%,$(CC_VERSION) $(RUSTC_LLVM_VERSION))))
+# Disable value profiling when:
+# (RUSTC_LLVM_VERSION < 12 and CC_VERSION >= 12) or (RUSTC_LLVM_VERSION >= 12 and CC_VERSION < 12)
+rust_pgo_flags += -C llvm-args=--disable-vp=true
+endif
 # The C compiler may be passed extra llvm flags for PGO that we also want to pass to rust as well.
 # In PROFILE_GEN_CFLAGS, they look like "-mllvm foo", and we want "-C llvm-args=foo", so first turn
 # "-mllvm foo" into "-mllvm:foo" so that it becomes a unique argument, that we can then filter for,