Optimize rebuilds by reducing $(shell) usage

$(shell) isn't particularly fast in Kati, and they have to be executed
both when reading the makefiles and determining whether the ninja file
needs to be regenerated.

Right now, the regen time is mostly hidden because we run them in
parallel. We've also configured it to ignore any commands that contain
"echo", "date", or the output directory. That happens to remove most
commands that contain side effects, so running them in parallel is fine.

But the side effects contain some important things, like the clean up
necessary when switching products. So I'm removing those filters, and
then we'll need to run the shell commands in sequence, since there will
be side-effects. That makes regen take longer though, so use pure-Make
implementations instead of $(shell) where possible.

This set of changes reduces aosp/master aosp_arm64-eng build $(shell)
usage and time by 2/3:

*kati*: func shell time: 3.135095 / 709
*kati*: func shell time: 1.067331 / 236

Bug: 30947985
Test: Manual test lines for math functions
Test: Compare build-aosp_arm64.ninja before/after
Change-Id: I4fc9d6318957992921972994f277c17918e7e1eb

core/binary.mk

@@ -103,9 +103,7 @@ ifdef LOCAL_SDK_VERSION
   # missing API levels to existing ones where necessary, but we're not doing
   # that for the generated libraries. Clip the API level to the minimum where
   # appropriate.
-  my_ndk_api := \
-    $(shell if [ $(LOCAL_SDK_VERSION) -lt $(my_min_sdk_version) ]; then \
-        echo $(my_min_sdk_version); else echo $(LOCAL_SDK_VERSION); fi)
+  my_ndk_api := $(call math_max,$(LOCAL_SDK_VERSION),$(my_min_sdk_version))
 
   # Traditionally this has come from android/api-level.h, but with the libc
   # headers unified it must be set by the build system since we don't have

core/definitions.mk

@@ -717,15 +717,6 @@ define jack-lib-files
 $(foreach lib,$(1),$(call _jack-lib-full-classes,$(lib),$(2)))
 endef
 
-###########################################################
-## Run rot13 on a string
-## $(1): the string.  Must be one line.
-###########################################################
-define rot13
-$(shell echo $(1) | tr 'a-zA-Z' 'n-za-mN-ZA-M')
-endef
-
-
 ###########################################################
 ## Returns true if $(1) and $(2) are equal.  Returns
 ## the empty string if they are not equal.
@@ -3169,6 +3160,55 @@ endif
 endif
 endef
 
+###########################################################
+# Basic math functions for positive integers <= 100
+#
+# (SDK versions for example)
+###########################################################
+__MATH_NUMBERS :=  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 \
+                  21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 \
+                  41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 \
+                  61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 \
+                  81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
+
+# Returns true if $(1) is a positive integer <= 100, otherwise returns nothing.
+define math_is_number
+$(strip \
+  $(if $(1),,$(error Argument missing)) \
+  $(if $(word 2,$(1)),$(error Multiple words in a single argument: $(1))) \
+  $(if $(filter $(1),$(__MATH_NUMBERS)),true))
+endef
+
+#$(warning true == $(call math_is_number,2))
+#$(warning == $(call math_is_number,foo))
+#$(call math_is_number,1 2)
+#$(call math_is_number,no 2)
+
+define _math_check_valid
+$(if $(call math_is_number,$(1)),,$(error Only positive integers <= 100 are supported (not $(1))))
+endef
+
+#$(call _math_check_valid,0)
+#$(call _math_check_valid,1)
+#$(call _math_check_valid,100)
+#$(call _math_check_valid,101)
+#$(call _math_check_valid,)
+#$(call _math_check_valid,1 2)
+
+# Returns the greater of $1 or $2.
+# If $1 or $2 is not a positive integer <= 100, then an error is generated.
+define math_max
+$(strip $(call _math_check_valid,$(1)) $(call _math_check_valid,$(2)) \
+  $(lastword $(filter $(1) $(2),$(__MATH_NUMBERS))))
+endef
+
+#$(call math_max)
+#$(call math_max,1)
+#$(call math_max,1 2,3)
+#$(warning 1 == $(call math_max,1,1))
+#$(warning 42 == $(call math_max,5,42))
+#$(warning 42 == $(call math_max,42,5))
+
 ###########################################################
 ## Other includes
 ###########################################################

core/product.mk

@@ -301,7 +301,7 @@ _product_stash_var_list += \
 #
 define stash-product-vars
 $(foreach v,$(_product_stash_var_list), \
-        $(eval $(strip $(1))_$(call rot13,$(v)):=$$($$(v))) \
+        $(eval $(strip $(1))_rot26_$(v):=$$($$(v))) \
  )
 endef
 
@@ -313,7 +313,7 @@ define assert-product-vars
 $(strip \
   $(eval changed_variables:=)
   $(foreach v,$(_product_stash_var_list), \
-    $(if $(call streq,$($(v)),$($(strip $(1))_$(call rot13,$(v)))),, \
+    $(if $(call streq,$($(v)),$($(strip $(1))_rot26_$(v))),, \
         $(eval $(warning $(v) has been modified: $($(v)))) \
         $(eval $(warning previous value: $($(strip $(1))_$(call rot13,$(v))))) \
         $(eval changed_variables := $(changed_variables) $(v))) \