Move to the more automated gem5-bench benchmarking script.

Enable everything in the toolchain in preparation to future benchmarking
to prevent future rebuilds, notably C++, Fortran and LTO support.

Document compiler optimizations for benchmarking.

Document graph-build for monitoring build times.

README.adoc

@@ -1533,24 +1533,24 @@ https://stackoverflow.com/questions/48944587/how-to-count-the-number-of-cpu-cloc
 Let's benchmark https://en.wikipedia.org/wiki/Dhrystone[Dhrystone] which Buildroot provides:
 
 ....
-./run -a arm -e 'init=/eval.sh - lkmc_eval="m5 checkpoint;m5 dumpstats;dhrystone 1000;m5 exit"' -g
-./gem5-cycles
+./gem5-bench dhrystone 1000
 ....
 
-`./gem5-cycles` outputs the approximate number of CPU cycles it took Dhrystone to run. A few possible problems are:
+This initial run generates a <<gem5-checkpoint,checkpoint>> after the kernel boots and before running the benchmark.
+
+Then we can speed up further benchmark runs by skipping the Linux kernel boot:
+
+....
+./gem5-bench -r dhrystone 1000
+....
+
+These commands output the approximate number of CPU cycles it took Dhrystone to run. A few possible problems are:
 
 * when we do `m5 dumpstats`, there is some time passed before the `exec` system call returns and the actual benchmark starts
 * the benchmark outputs to stdout, which means so extra cycles in addition to the actual computation. But TODO: how to get the output to check that it is correct without such IO cycles?
 
 Those problems should be insignificant if the benchmark runs for long enough however.
 
-We can then speed up further benchmark runs by skipping the Linux kernel boot:
-
-....
-./run -a arm -e 'init=/eval.sh - lkmc_eval="m5 dumpstats;dhrystone 1000;m5 exit"' -g -- -r 1
-./gem5-cycles
-....
-
 TODO: the cycle counts on the original run and the one with checkpoint restore differ slightly. Why? Multiple checkpoint restores give the same results however.
 
 Now you can play a fun little game with your friends:
@@ -1575,6 +1575,26 @@ Each time we run `m5 dumpstats`, a section with the following format is added to
 
 TODO: diff out all the stats, not just `system.cpu.numCycles`.
 
+====== Enable compiler optimizations
+
+If you are benchmarking compiled programs instead of hand written assembly, remember that we configure Buildroot to disable optimizations by default with:
+
+....
+BR2_OPTIMIZE_0=y
+....
+
+to improve the debugging experience.
+
+You will likely want to change that to:
+
+....
+BR2_OPTIMIZE_3=y
+....
+
+and do a full rebuild.
+
+TODO is it possible to compile a single package with optimizations enabled? In any case, this wouldn't be very representative, since calls to an unoptimized libc will also have an impact on performance. Kernel-wise it should be fine though, since the kernel requires `O=2`.
+
 ===== GEM5 kernel boot command line arguments
 
 Analogous <<kernel-boot-command-line-arguments,to QEMU>>:
@@ -2231,6 +2251,19 @@ diff .config.olg .config
 
 Copy and paste the diff additions to `buildroot_config_fragment`.
 
+==== What is making my build so slow?
+
+....
+cd buildroot/output.x86_64~
+make graph-build
+xdg-open graphs/build.pie-packages.pdf
+....
+
+Our phylosophy is:
+
+* if something adds little to the build time, build it in by default
+* otherwise, make it optional
+
 === About
 
 This project is for people who want to learn and modify low level system components:

buildroot_config_fragment

@@ -1,3 +1,7 @@
+BR2_ENABLE_LOCALE=y
+BR2_GCC_ENABLE_GRAPHITE=y
+BR2_GCC_ENABLE_LTO=y
+BR2_GCC_ENABLE_OPENMP=y
 BR2_GLOBAL_PATCH_DIR="../global_patch_dir"
 BR2_PACKAGE_BUSYBOX_CONFIG_FRAGMENT_FILES="../busybox_config_fragment"
 BR2_PACKAGE_DHRYSTONE=y
@@ -12,6 +16,8 @@ BR2_ROOTFS_POST_IMAGE_SCRIPT="../rootfs_post_image_script"
 BR2_ROOTFS_USERS_TABLES="../user_table"
 BR2_TARGET_ROOTFS_CPIO=y
 BR2_TARGET_ROOTFS_EXT2=y
+BR2_TOOLCHAIN_BUILDROOT_CXX=y
+BR2_TOOLCHAIN_BUILDROOT_FORTRAN=y
 BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
 
 # Host GDB

gem5-bench

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+replay=false
+while getopts r OPT; do
+  case "$OPT" in
+    r)
+      replay=true
+      ;;
+  esac
+done
+shift "$(($OPTIND - 1))"
+bench="$@"
+if "$replay"; then
+  ./run -a arm -e 'init=/eval.sh - lkmc_eval="m5 resetstats;'"$bench"';m5 exit"' -g -- -r 1
+else
+  ./run -a arm -e 'init=/eval.sh - lkmc_eval="m5 checkpoint;m5 resetstats;'"$bench"';m5 exit"' -g
+fi
+awk '/^system.cpu.numCycles /{ print $2 }' m5out/stats.txt

gem5-cycles

@@ -1,2 +0,0 @@
-#!/usr/bin/env bash
-grep numCycles m5out/stats.txt | awk '{t0 = $2; getline; print $2 - t0; exit;}'