[PATCH v2 1/5] kbuild: add test-{le,ge,lt,gt} macros

* [PATCH v2 1/5] kbuild: add test-{le,ge,lt,gt} macros
@ 2022-11-23 15:18 ` Masahiro Yamada
  0 siblings, 0 replies; 20+ messages in thread
From: Masahiro Yamada @ 2022-11-23 15:18 UTC (permalink / raw)
  To: linux-kbuild
  Cc: linux-kernel, Masahiro Yamada, Albert Ou, Borislav Petkov,
	Dave Hansen, H. Peter Anvin, Ingo Molnar, Nathan Chancellor,
	Nick Desaulniers, Nicolas Schier, Palmer Dabbelt, Paul Walmsley,
	Thomas Gleixner, Tom Rix, linux-riscv, llvm, x86

Because GNU Make is only able to handle strings, it is very hard to
perform arighmetic in Makefiles.

When we compare two integers, we invokes shell. One example is in the
top Makefile:

  ifeq ($(shell test $(CONFIG_LLD_VERSION) -lt 130000; echo $$?),0)

This is more expensive than using built-in functions since it forks a
process.

If we know the two have the same number of digits, we can do better.

This commit adds four macros, test-le, test-ge, test-lt, test-gt.

$(call test-lt, A, B) is evaluated to 'y' if A is less than B, or
empty otherwise. This will replace $(call shell test A -lt B).

Again, the limitation is that A and B must have the same number of
digits because these macros are based on $(sort ) function.

  $(call test-lt, 1, 9)    -->  y        (Works!)
  $(call test-lt, 10, 9)   -->  y        (Not work...)

To make the latter work, you need to add '0' prefix to align the number
of digits:

  $(call test-lt, 10, 09)  -->  empty    (Works!)

Actually, we can live with this limitation in many places. As for the
example above, we know $(CONFIG_LLD_VERSION) is 6-digits because the
minimal supported version of LLVM is 11.0.0.

So, the shell invocation can be replaced with more efficient code:

  ifeq ($(call test-lt, $(CONFIG_LLD_VERSION), 130000),y)

Of course, this assumption will break when LLVM 100 is released, but it
will be far in the future.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

(no changes since v1)

 Makefile               |  2 +-
 arch/riscv/Makefile    |  2 +-
 arch/x86/Makefile      |  2 +-
 scripts/Kbuild.include | 10 ++++++++++
 4 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 6f846b1f2618..eb80332f7b51 100644
--- a/Makefile
+++ b/Makefile
@@ -986,7 +986,7 @@ KBUILD_LDFLAGS += -mllvm -import-instr-limit=5
 # Check for frame size exceeding threshold during prolog/epilog insertion
 # when using lld < 13.0.0.
 ifneq ($(CONFIG_FRAME_WARN),0)
-ifeq ($(shell test $(CONFIG_LLD_VERSION) -lt 130000; echo $$?),0)
+ifeq ($(call test-lt, $(CONFIG_LLD_VERSION), 130000),y)
 KBUILD_LDFLAGS	+= -plugin-opt=-warn-stack-size=$(CONFIG_FRAME_WARN)
 endif
 endif
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 0d13b597cb55..faf2c2177094 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -37,7 +37,7 @@ else
 endif
 
 ifeq ($(CONFIG_LD_IS_LLD),y)
-ifeq ($(shell test $(CONFIG_LLD_VERSION) -lt 150000; echo $$?),0)
+ifeq ($(call test-lt, $(CONFIG_LLD_VERSION), 150000),y)
 	KBUILD_CFLAGS += -mno-relax
 	KBUILD_AFLAGS += -mno-relax
 ifndef CONFIG_AS_IS_LLVM
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 415a5d138de4..e72c7a49cd59 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -211,7 +211,7 @@ endif
 KBUILD_LDFLAGS += -m elf_$(UTS_MACHINE)
 
 ifdef CONFIG_LTO_CLANG
-ifeq ($(shell test $(CONFIG_LLD_VERSION) -lt 130000; echo $$?),0)
+ifeq ($(call test-lt, $(CONFIG_LLD_VERSION), 130000),y)
 KBUILD_LDFLAGS	+= -plugin-opt=-stack-alignment=$(if $(CONFIG_X86_32),4,8)
 endif
 endif
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index cbe28744637b..9996f34327cb 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -11,6 +11,16 @@ space   := $(empty) $(empty)
 space_escape := _-_SPACE_-_
 pound := \#
 
+###
+# Comparison macros.
+# Usage: $(call test-le, A, B)
+# works like shell's "test A -le B", but A and B must have the same number of
+# digits since it is just ASCII sort.
+test-le = $(if $(filter $1, $(firstword $(sort $1 $2))),y)
+test-ge = $(call test-le, $2, $1)
+test-lt = $(if $(filter-out $2, $(firstword $(sort $1 $2))),y)
+test-gt = $(call test-lt, $2, $1)
+
 ###
 # Name of target with a '.' as filename prefix. foo/bar.o => foo/.bar.o
 dot-target = $(dir $@).$(notdir $@)
-- 
2.34.1


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