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[2/7] Don't disable SSE in x86-64 ld.so

Message ID CAMe9rOpNOzCWTfH6V+5WU3OM2VG+trywzf_Zogm0GCHZOPGz0g@mail.gmail.com
State New
Headers show

Commit Message

H.J. Lu Aug. 25, 2015, 9:41 p.m. UTC 
On Tue, Aug 25, 2015 at 2:31 PM, Roland McGrath <roland@hack.frob.com> wrote:
> I meant a comment for the CFLAGS-.os setting, but close enough.
>
> However, the x86_64/Makefile bit needs a comment for its CFLAGS-.os
> setting that explains why -mno-mmx is necessary in ld.so code.

We use -mno-mmx on i386 since mm registers are passed to pass
__m64 arguments.  On x86-64, I can't think of a good reason to use
MMX in ld.so.

Here is the updated patch.

Comments

Roland McGrath Aug. 25, 2015, 9:48 p.m. UTC | #1 
> We use -mno-mmx on i386 since mm registers are passed to pass
> __m64 arguments.  On x86-64, I can't think of a good reason to use
> MMX in ld.so.

The patch is OK now since -mno-mmx was the status quo ante for x86_64.

But the rationale for -mno-mmx is being stated in a backwards fashion.
We should add special options only if there is a good reason to have
them.  If you just don't think it's optimal to use MMX, then that is not
a good reason to use -mno-mmx; it's a good reason to make some compiler
change to improve optimization.  If there is a specific, known,
pathological optimization issue, then there should be a GCC bug filed
for that and the comments in the makefile should point to that bug.  If
there is no such reason to specifically use -mno-mmx, then we should
drop it.
H.J. Lu Aug. 25, 2015, 9:55 p.m. UTC | #2 
On Tue, Aug 25, 2015 at 2:48 PM, Roland McGrath <roland@hack.frob.com> wrote:
>> We use -mno-mmx on i386 since mm registers are passed to pass
>> __m64 arguments.  On x86-64, I can't think of a good reason to use
>> MMX in ld.so.
>
> The patch is OK now since -mno-mmx was the status quo ante for x86_64.
>
> But the rationale for -mno-mmx is being stated in a backwards fashion.
> We should add special options only if there is a good reason to have
> them.  If you just don't think it's optimal to use MMX, then that is not
> a good reason to use -mno-mmx; it's a good reason to make some compiler
> change to improve optimization.  If there is a specific, known,
> pathological optimization issue, then there should be a GCC bug filed
> for that and the comments in the makefile should point to that bug.  If
> there is no such reason to specifically use -mno-mmx, then we should
> drop it.

Here is a partial list of MMX bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48397
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47759
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40983
diff mbox

Patch

From f65ffc892e43fb90ed3235a30e3cecb0faa3f0d6 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Tue, 28 Jul 2015 18:56:18 -0700
Subject: [PATCH] Don't disable SSE in x86-64 ld.so

Since x86-64 ld.so preserves vector registers now, we can use SSE in
x86-64 ld.so.  We should run tst-ld-sse-use.sh only on i386.

	* sysdeps/x86/Makefile [$(subdir) == elf] (CFLAGS-.os,
	tests-special, $(objpfx)tst-ld-sse-use.out): Moved to ...
	* sysdeps/i386/Makefile [$(subdir) == elf] (CFLAGS-.os,
	tests-special, $(objpfx)tst-ld-sse-use.out): Here.  Update
	comments.
	* sysdeps/x86_64/Makefile [$(subdir) == elf] (CFLAGS-.os): Add
	-mno-mmx for $(all-rtld-routines).
	* sysdeps/x86/tst-ld-sse-use.sh: Moved to ...
	* sysdeps/i386/tst-ld-sse-use.sh: Here.  Replace x86-64 with
	i386.
---
 sysdeps/i386/Makefile                   | 14 ++++++++++++++
 sysdeps/{x86 => i386}/tst-ld-sse-use.sh |  2 +-
 sysdeps/x86/Makefile                    | 11 -----------
 sysdeps/x86_64/Makefile                 |  4 ++++
 4 files changed, 19 insertions(+), 12 deletions(-)
 rename sysdeps/{x86 => i386}/tst-ld-sse-use.sh (97%)

diff --git a/sysdeps/i386/Makefile b/sysdeps/i386/Makefile
index 717d8e7..168512f 100644
--- a/sysdeps/i386/Makefile
+++ b/sysdeps/i386/Makefile
@@ -83,3 +83,17 @@  endif
 ifeq ($(subdir),csu)
 gen-as-const-headers += tlsdesc.sym
 endif
+
+ifeq ($(subdir),elf)
+# Make sure no code in ld.so uses mm/xmm/ymm/zmm registers on i386 since
+# the first 3 mm/xmm/ymm/zmm registers are used to pass vector parameters
+# which must be preserved.
+CFLAGS-.os += $(if $(filter $(@F),$(patsubst %,%.os,$(all-rtld-routines))),\
+		   -mno-sse -mno-mmx)
+
+tests-special += $(objpfx)tst-ld-sse-use.out
+$(objpfx)tst-ld-sse-use.out: ../sysdeps/i386/tst-ld-sse-use.sh $(objpfx)ld.so
+	@echo "Checking ld.so for SSE register use.  This will take a few seconds..."
+	$(BASH) $< $(objpfx) '$(NM)' '$(OBJDUMP)' '$(READELF)' > $@; \
+	$(evaluate-test)
+endif
diff --git a/sysdeps/x86/tst-ld-sse-use.sh b/sysdeps/i386/tst-ld-sse-use.sh
similarity index 97%
rename from sysdeps/x86/tst-ld-sse-use.sh
rename to sysdeps/i386/tst-ld-sse-use.sh
index 839de18..85a0651 100755
--- a/sysdeps/x86/tst-ld-sse-use.sh
+++ b/sysdeps/i386/tst-ld-sse-use.sh
@@ -1,5 +1,5 @@ 
 #! /bin/bash
-# Make sure no code in ld.so uses xmm/ymm/zmm registers on x86-64.
+# Make sure no code in ld.so uses xmm/ymm/zmm registers on i386.
 # Copyright (C) 2009-2015 Free Software Foundation, Inc.
 # This file is part of the GNU C Library.
 
diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
index c262fdf..0de4f42 100644
--- a/sysdeps/x86/Makefile
+++ b/sysdeps/x86/Makefile
@@ -1,14 +1,3 @@ 
-ifeq ($(subdir),elf)
-CFLAGS-.os += $(if $(filter $(@F),$(patsubst %,%.os,$(all-rtld-routines))),\
-		   -mno-sse -mno-mmx)
-
-tests-special += $(objpfx)tst-ld-sse-use.out
-$(objpfx)tst-ld-sse-use.out: ../sysdeps/x86/tst-ld-sse-use.sh $(objpfx)ld.so
-	@echo "Checking ld.so for SSE register use.  This will take a few seconds..."
-	$(BASH) $< $(objpfx) '$(NM)' '$(OBJDUMP)' '$(READELF)' > $@; \
-	$(evaluate-test)
-endif
-
 ifeq ($(subdir),csu)
 gen-as-const-headers += cpu-features-offsets.sym rtld-global-offsets.sym
 endif
diff --git a/sysdeps/x86_64/Makefile b/sysdeps/x86_64/Makefile
index e50bcad..6c28318 100644
--- a/sysdeps/x86_64/Makefile
+++ b/sysdeps/x86_64/Makefile
@@ -19,6 +19,10 @@  gen-as-const-headers += locale-defines.sym
 endif
 
 ifeq ($(subdir),elf)
+# There is no good reason to use MMX in x86-64 ld.so with GCC.
+CFLAGS-.os += $(if $(filter $(@F),$(patsubst %,%.os,$(all-rtld-routines))),\
+		   -mno-mmx)
+
 sysdep-dl-routines += tlsdesc dl-tlsdesc
 
 tests += ifuncmain8
-- 
2.4.3