bazel: simplify lto flags

Introduce --config=lto mixin which sets -flto=thin for C++ compile and
link. Remove src/v/redpanda specific handling of the link flag, we
pass this to all links now. This is effectively a no-op as lld will
use LTO mode when inputs have only IR anyway.

You can mix --config=lto with other build modes (most usefully, release)
even without PGO.

Author: travisdowns

.bazelrc

@@ -132,6 +132,19 @@ build:san-all  --//bazel:sanitizer_mode=all
 build:san-all     --copt=-fsanitize=address,undefined,vptr,function,alignment
 build:san-all  --linkopt=-fsanitize=address,undefined,vptr,function,alignment
 
+# LTO mixin: enable LTO for C++ and Rust. Usually mixed in with
+# PGO + release, but should work with any.
+build:lto --copt -flto=thin
+build:lto --linkopt -flto=thin
+build:lto --@rules_rust//rust/settings:lto=fat
+build:lto --@rules_rust//rust/settings:extra_rustc_flag=-Ccodegen-units=1
+# I've seen others claim these flags help with LTO and the final binary, but these also
+# require a specific version of clang and I don't really want to tie rustc version with
+# clang version.
+# build:lto --@rules_rust//rust/settings:extra_rustc_flag=-Clinker-plugin-lto
+# build:lto --@rules_rust//rust/settings:extra_rustc_flag=-Cembed-bitcode=yes
+# build:lto --@rules_rust//rust/settings:extra_rustc_flag=-Clink-arg=-fuse-ld=lld
+
 ################################################
 # PRIMARY BUILD MODES
 #
@@ -251,23 +264,10 @@ build:release --host_cxxopt=-U_LIBCPP_HARDENING_MODE
 build:release --cxxopt=-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE
 build:release --host_cxxopt=-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE
 
-# Separate PGO configs to be combined with release
-
-# Enable LTO only in PGO build to save time in other builds
-build:pgo-base --//src/v/redpanda:lto=True
-build:pgo-base --copt -flto=thin
-build:pgo-base --@rules_rust//rust/settings:lto=fat
-build:pgo-base --@rules_rust//rust/settings:extra_rustc_flag=-Ccodegen-units=1
-# I've seen others claim these flags help with LTO and the final binary, but these also
-# require a specific version of clang and I don't really want to tie rustc version with
-# clang version.
-# build:pgo-base --@rules_rust//rust/settings:extra_rustc_flag=-Clinker-plugin-lto
-# build:pgo-base --@rules_rust//rust/settings:extra_rustc_flag=-Cembed-bitcode=yes
-# build:pgo-base --@rules_rust//rust/settings:extra_rustc_flag=-Clink-arg=-fuse-ld=lld
 
 # Instrumentation/training step
 # Empty arg is intentional, we provide the path via env var at run time
-build:pgo-instrument --config=pgo-base --fdo_instrument=
+build:pgo-instrument --config=lto --fdo_instrument=
 
 # Final/Optimization build step
 
@@ -278,7 +278,7 @@ build:pgo-instrument --config=pgo-base --fdo_instrument=
 
 # bazel seems to pass -fprofile-correction unconditionally but that only exists
 # in gcc so need to make clang ignore it
-build:pgo-optimize --config=pgo-base --copt -Wno-ignored-optimization-argument
+build:pgo-optimize --config=lto --copt -Wno-ignored-optimization-argument
 
 
 build:stamp --stamp --workspace_status_command=./bazel/stamp_vars.sh

src/v/redpanda/BUILD

@@ -124,20 +124,6 @@ redpanda_cc_library(
     ],
 )
 
-# Enable or disable (thin) LTO for the redpanda binary (only)
-bool_flag(
-    name = "lto",
-    build_setting_default = False,
-)
-
-config_setting(
-    name = "use_lto",
-    flag_values = {
-        ":lto": "true",
-    },
-    visibility = ["//visibility:public"],
-)
-
 # Enable or disable --emit-relocs for the redpanda binary (only)
 bool_flag(
     name = "emit_relocs",
@@ -158,14 +144,6 @@ redpanda_cc_binary(
         "main.cc",
     ],
     linkopts =
-        select({
-            ":use_lto": [
-                "-flto=thin",
-                "-ffat-lto-objects",
-            ],
-            "//conditions:default": [
-            ],
-        }) +
         select({
             ":use_emit_relocs": [
                 "-Wl,--emit-relocs",