[PATCH] Try LTO partial linking. (Was: Speed of compiling gimple-match.c)

["Martin Liška" <mliska@suse.cz>]

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Hello.

I've got a patch candidate that leverages partial linking for a couple of selected object files.

I'm sending make all-host- jX results for my machine:

before: 3m18s (user 32m52s)
https://gist.githubusercontent.com/marxin/223890df4d8d8e490b6b2918b77dacad/raw/1dd5eae5001295ba0230a689f7edc67284c9b742/gcc-all-host.svg

after: 2m57m (user 35m)
https://gist.githubusercontent.com/marxin/223890df4d8d8e490b6b2918b77dacad/raw/d659b2187cf622167841efbbe6bc93cb33855fa9/gcc-all-host-partial-lto.svg

One can utilize it with:
make -j16 all-host PARTIAL_LTO=1

@Segher, Andrew: Can you please measure time improvement for your slow bootstrap?
One can also tweak --param=lto-partitions=16 param value.

Thoughts?
Thanks,
Martin

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From 85228e612610c0e4b0324f6bebc84ef7c0211c4a Mon Sep 17 00:00:00 2001
From: Martin Liska <mliska@suse.cz>
Date: Thu, 20 May 2021 14:29:35 +0200
Subject: [PATCH] Try LTO partial linking.

---
 gcc/Makefile.in | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 1164554e6d6..f76bcea66f5 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -220,7 +220,9 @@ libgcov-util.o-warn = -Wno-error
 libgcov-driver-tool.o-warn = -Wno-error
 libgcov-merge-tool.o-warn = -Wno-error
 gimple-match.o-warn = -Wno-unused
+gimple-match-lto.o-warn = -Wno-unused
 generic-match.o-warn = -Wno-unused
+generic-match-lto.o-warn = -Wno-unused
 dfp.o-warn = -Wno-strict-aliasing
 
 # All warnings have to be shut off in stage1 if the compiler used then
@@ -1282,12 +1284,10 @@ ANALYZER_OBJS = \
 # will build them sooner, because they are large and otherwise tend to be
 # the last objects to finish building.
 OBJS = \
-	gimple-match.o \
-	generic-match.o \
+	common-base.a \
 	insn-attrtab.o \
 	insn-automata.o \
 	insn-dfatab.o \
-	insn-emit.o \
 	insn-extract.o \
 	insn-latencytab.o \
 	insn-modes.o \
@@ -1295,7 +1295,6 @@ OBJS = \
 	insn-output.o \
 	insn-peep.o \
 	insn-preds.o \
-	insn-recog.o \
 	insn-enums.o \
 	ggc-page.o \
 	adjust-alignment.o \
@@ -2627,6 +2626,29 @@ s-match: build/genmatch$(build_exeext) $(srcdir)/match.pd cfn-operators.pd
 	    					generic-match.c
 	$(STAMP) s-match
 
+ifdef PARTIAL_LTO
+LTO_LINKER_FLAGS = -flto=auto --param=lto-partitions=16 -flinker-output=nolto-rel -r
+LTO_FLAGS = -flto
+
+gimple-match-lto.o: gimple-match.c $(TARGET_H)
+	$(COMPILE) $< $(LTO_FLAGS)
+generic-match-lto.o: generic-match.c $(TARGET_H)
+	$(COMPILE) $< $(LTO_FLAGS)
+insn-recog-lto.o: insn-recog.c
+	$(COMPILE) $< $(LTO_FLAGS)
+insn-emit-lto.o: insn-emit.c
+	$(COMPILE) $< $(LTO_FLAGS)
+
+common-base.a: gimple-match-lto.o generic-match-lto.o insn-recog-lto.o insn-emit-lto.o
+	-rm -rf $@
+	$(LINKER) $^ $(LTO_LINKER_FLAGS) -o common-base.o
+	$(AR) $(AR_FLAGS)T $@ common-base.o
+else
+common-base.a: gimple-match.o generic-match.o insn-recog.o insn-emit.o
+	-rm -rf $@
+	$(AR) $(AR_FLAGS)T $@ $^
+endif
+
 GTFILES = $(CPPLIB_H) $(srcdir)/input.h $(srcdir)/coretypes.h \
   $(host_xm_file_list) \
   $(tm_file_list) $(HASHTAB_H) $(SPLAY_TREE_H) $(srcdir)/bitmap.h \
-- 
2.31.1