From: Han Wang <nesthanwang@google.com>

AutoFDO profiles are commonly collected on one kernel revision and
reused across subsequent commits or stable merges before a fresh
profile is generated. While AutoFDO uses function-relative line
numbers to tolerate edits above a function, adding or removing lines
inside a function (or an inlined helper) shifts all subsequent
relative line offsets within that function. Without stale profile
salvaging, shifted callsites fail to match their nested inlinee
profiles in SampleProfileLoader (dropping profile-guided inlining for
those callees) and basic block sample counts are misattributed to the
wrong instructions, degrading branch probability estimation and
machine function splitting.

Enable LLVM stale profile salvaging (-salvage-stale-profile, supported
since LLVM 17) for C, Rust, and ThinLTO builds when
CLANG_AUTOFDO_PROFILE is specified. This instructs SampleProfileMatcher
to use lexical callsite anchors within functions to fuzzy-match and
remap shifted line offsets when querying the profile.

While -salvage-stale-profile is not enabled by default in LLVM due to
the minor compile-time/memory cost of sequence-matching callsite
anchors (which is unnecessary when profiles are freshly generated on
the exact commit being built) and the heuristic nature of fuzzy
matching on heavily rewritten functions, enabling it makes sense for
the kernel: hardware PMU profiles are rarely regenerated on every
commit, and anchoring on callsite names is strictly more resilient
than blind line-offset lookup when functions are edited.

As a side benefit, remapping shifted line offsets also avoids feeding
mismatched weights from stale profiles into MIRProfileLoader's flow
inference solver (applyFlowInference), working around a compiler bug
[1] where a small intra-function line shift (such as in
__vm_normal_page() in mm/memory.c) can cause applyFlowInference to
spend 30+ minutes compiling unmap_page_range().

Link: https://github.com/ClangBuiltLinux/linux/issues/2188 [1]
Signed-off-by: Han Wang <nesthanwang@google.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---
Much of the commit-message justification here is AI-generated. Sorry!
:( Folks who know better, feel free to call bullshit.

Honestly, this patch came about because of AI analysis compiles taking
30 minutes to run (see the clang bug [1]). It seems to work around the
problem. Obviously, we should fix the compiler. That being said, upon
further analysis it seemed like adding this flag was beneficial
anyway, so I'm posting it upstream. If folks disagree, we can simply
drop this patch.

 scripts/Makefile.autofdo | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
index 57a3aba6ffc2..3b1e65a13ffa 100644
--- a/scripts/Makefile.autofdo
+++ b/scripts/Makefile.autofdo
@@ -13,13 +13,16 @@ endif
 ifdef CLANG_AUTOFDO_PROFILE
   CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -ffunction-sections
   CFLAGS_AUTOFDO_CLANG += -fsplit-machine-functions
+  CFLAGS_AUTOFDO_CLANG += -mllvm -salvage-stale-profile
   RUSTFLAGS_AUTOFDO_CLANG += -Zprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -Zfunction-sections=y
   RUSTFLAGS_AUTOFDO_CLANG += -Cllvm-args=-split-machine-functions
+  RUSTFLAGS_AUTOFDO_CLANG += -Cllvm-args=-salvage-stale-profile
 endif
 
 ifdef CONFIG_LTO_CLANG_THIN
   ifdef CLANG_AUTOFDO_PROFILE
     KBUILD_LDFLAGS += --lto-sample-profile=$(CLANG_AUTOFDO_PROFILE)
+    KBUILD_LDFLAGS += --mllvm=-salvage-stale-profile
   endif
   KBUILD_LDFLAGS += --mllvm=-enable-fs-discriminator=true --mllvm=-improved-fs-discriminator=true -plugin-opt=thinlto
   KBUILD_LDFLAGS += $(call ld-option,-plugin-opt=-function-splitting=all,-plugin-opt=-split-machine-functions)
-- 
2.56.0.455.g6c5a503b38-goog