linux-amd64: park -fbasic-block-sections=all until Propeller ships

Per-BB sections only pay off when paired with a Propeller-style symbol-
ordering file at the consumer's final link; without that the flag is
pure overhead (codesize growth + per-block section relocations). Move
to labs.disabled.txt and re-enable once we're sure we ship Propeller in
the linux-amd64 pipeline. Note that the `=all` form is x86-ELF-only, so
when re-enabled the flag belongs in linux-amd64.txt, not linux.txt.

Author: sgammon

labs.disabled.txt

@@ -64,3 +64,14 @@
 # when the linux-amd64 deployment floor moves to Tiger Lake+ / Zen 4+
 # and the userspace runtime contract is ready to require CET support.
 -fcf-protection=full
+#
+# Per-basic-block sections — prerequisite for a Propeller-style
+# symbol-ordering pass at the consumer's final link, where hot blocks
+# get reordered across function boundaries to shrink the icache
+# working set. Without the ordering file the flag is pure overhead
+# (codesize growth + per-block section relocations), so we keep it
+# disabled until Propeller is actually wired into the linux-amd64
+# pipeline. clang's `=all` form is x86 ELF only — AArch64 clang
+# rejects `all`, darwin clang doesn't implement BB sections — so when
+# re-enabled it belongs in linux-amd64.txt, not linux.txt.
+-fbasic-block-sections=all

linux-amd64.txt

@@ -44,15 +44,11 @@
 # linux-amd64 deployment floor moves to Tiger Lake+ / Zen 4+ and the
 # userspace runtime contract is ready to require CET support.
 
-# Emit per-basic-block sections so a future Propeller pass can feed a
-# symbol-ordering file into the linker and reorder hot blocks across
-# function boundaries. Without that ordering file the linker keeps
-# blocks in source order and the flag is overhead, but the per-BB
-# sections are a prerequisite for Propeller, so we eat the cost now to
-# avoid a rebuild-the-world later.
-#
-# clang's `=all` form is gated to x86 ELF; AArch64 clang on Alpine
-# rejects `all` ('invalid value all') even though it accepts `=labels`.
-# Darwin's clang doesn't implement BB sections at all. Pinning this to
-# linux-amd64 avoids both failure modes.
--fbasic-block-sections=all
+# NOTE: -fbasic-block-sections=all is parked in labs.disabled.txt.
+# Per-BB sections only pay off when paired with a Propeller-style
+# symbol-ordering file at the consumer's final link; without that the
+# flag is pure overhead (codesize growth + per-block section relocation
+# noise). Re-enable here once we are sure we ship Propeller in the
+# linux-amd64 pipeline. The flag is x86-ELF-only — AArch64 clang rejects
+# `=all` and darwin clang doesn't implement BB sections — so it stays
+# scoped to linux-amd64 when re-enabled.