From: Michael Matz <matz@suse.de> To: Segher Boessenkool <segher@kernel.crashing.org> Cc: Borislav Petkov <bp@alien8.de>, gcc@gcc.gnu.org, Richard Biener <rguenther@suse.de>, Nadav Amit <namit@vmware.com>, Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, x86@kernel.org, Masahiro Yamada <yamada.masahiro@socionext.com>, Sam Ravnborg <sam@ravnborg.org>, Alok Kataria <akataria@vmware.com>, Christopher Li <sparse@chrisli.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, "H. Peter Anvin" <hpa@zytor.com>, Jan Beulich <JBeulich@suse.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Juergen Gross <jgross@suse.com>, Kate Stewart <kstewart@linuxfoundation.org>, Kees Cook <keescook@chromium.org>, linux-sparse@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, Philippe Ombredanne <pombredanne@nexb.com>, Thomas Gleixner <tglx@linutronix.de>, virtualization@lists.linux-foundation.org, Linus Torvalds <torvalds@linux-foundation.org>, Chris Zankel <chris@zankel.net>, Max Filippov <jcmvbkbc@gmail.com>, linux-xtensa@linux-xtensa.org Subject: Re: PROPOSAL: Extend inline asm syntax with size spec Date: Sun, 7 Oct 2018 15:53:26 +0000 (UTC) [thread overview] Message-ID: <alpine.LSU.2.21.1810071534220.7867@wotan.suse.de> (raw) In-Reply-To: <20181007132228.GJ29268@gate.crashing.org> Hi Segher, On Sun, 7 Oct 2018, Segher Boessenkool wrote: > On Sun, Oct 07, 2018 at 11:18:06AM +0200, Borislav Petkov wrote: > > this is an attempt to see whether gcc's inline asm heuristic when > > estimating inline asm statements' cost for better inlining can be > > improved. > > GCC already estimates the *size* of inline asm, and this is required > *for correctness*. So any workaround that works against this will only > end in tears. You're right and wrong. GCC can't even estimate the size of mildly complicated inline asms right now, so your claim of it being necessary for correctness can't be true in this absolute form. I know what you try to say, but still, consider inline asms like this: insn1 .section bla insn2 .previous or invoke_asm_macro foo,bar in both cases GCCs size estimate will be wrong however you want to count it. This is actually the motivating example for the kernel guys, the games they play within their inline asms make the estimates be wildly wrong to a point it interacts with the inliner. > So I guess the real issue is that the inline asm size estimate for x86 > isn't very good (since it has to be pessimistic, and x86 insns can be > huge)? No, see above, even if we were to improve the size estimates (e.g. based on some average instruction size) the kernel examples would still be off because they switch sections back and forth, use asm macros and computed .fill directives and maybe further stuff. GCC will never be able to accurately calculate these sizes (without an built-in assembler which hopefully noone proposes). So, there is a case for extending the inline-asm facility to say "size is complicated here, assume this for inline decisions". > > Now, Richard suggested doing something like: > > > > 1) inline asm ("...") > > What would the semantics of this be? The size of the inline asm wouldn't be counted towards the inliner size limits (or be counted as "1"). > I don't like 2) either. But 1) looks interesting, depends what its > semantics would be? "Don't count this insn's size for inlining decisions", > maybe? TBH, I like the inline asm (...) suggestion most currently, but what if we want to add more attributes to asms? We could add further special keywords to the clobber list: asm ("...." : : : "cc,memory,inline"); sure, it might seem strange to "clobber" inline, but if we reinterpret the clobber list as arbitrary set of attributes for this asm, it'd be fine. > Another option is to just force inlining for those few functions where > GCC currently makes an inlining decision you don't like. Or are there > more than a few? I think the examples I saw from Boris were all indirect inlines: static inline void foo() { asm("large-looking-but-small-asm"); } static void bar1() { ... foo() ... } static void bar2() { ... foo() ... } void goo (void) { bar1(); } // bar1 should have been inlined So, while the immediate asm user was marked as always inline that in turn caused users of it to become non-inlined. I'm assuming the kernel guys did proper measurements that they _really_ get some non-trivial speed benefit by inlining bar1/bar2, but for some reasons (I didn't inquire) didn't want to mark them all as inline as well. Ciao, Michael.
WARNING: multiple messages have this Message-ID (diff)
From: Michael Matz <matz@suse.de> To: Segher Boessenkool <segher@kernel.crashing.org> Cc: Borislav Petkov <bp@alien8.de>, gcc@gcc.gnu.org, Richard Biener <rguenther@suse.de>, Nadav Amit <namit@vmware.com>, Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, x86@kernel.org, Masahiro Yamada <yamada.masahiro@socionext.com>, Sam Ravnborg <sam@ravnborg.org>, Alok Kataria <akataria@vmware.com>, Christopher Li <sparse@chrisli.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, "H. Peter Anvin" <hpa@zytor.com>, Jan Beulich <JBeulich@suse.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Juergen Gross <jgross@suse.com>, Kate Stewart <kstewart@linuxfoundation.org>, Kees Cook <keescook@chromium.org>, linux-sparse@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, Philippe Ombredanne <pombredanne@nexb.com>, Thomas Gleixner <tglx@linutroni> Subject: Re: PROPOSAL: Extend inline asm syntax with size spec Date: Sun, 7 Oct 2018 15:53:26 +0000 (UTC) [thread overview] Message-ID: <alpine.LSU.2.21.1810071534220.7867@wotan.suse.de> (raw) In-Reply-To: <20181007132228.GJ29268@gate.crashing.org> Hi Segher, On Sun, 7 Oct 2018, Segher Boessenkool wrote: > On Sun, Oct 07, 2018 at 11:18:06AM +0200, Borislav Petkov wrote: > > this is an attempt to see whether gcc's inline asm heuristic when > > estimating inline asm statements' cost for better inlining can be > > improved. > > GCC already estimates the *size* of inline asm, and this is required > *for correctness*. So any workaround that works against this will only > end in tears. You're right and wrong. GCC can't even estimate the size of mildly complicated inline asms right now, so your claim of it being necessary for correctness can't be true in this absolute form. I know what you try to say, but still, consider inline asms like this: insn1 .section bla insn2 .previous or invoke_asm_macro foo,bar in both cases GCCs size estimate will be wrong however you want to count it. This is actually the motivating example for the kernel guys, the games they play within their inline asms make the estimates be wildly wrong to a point it interacts with the inliner. > So I guess the real issue is that the inline asm size estimate for x86 > isn't very good (since it has to be pessimistic, and x86 insns can be > huge)? No, see above, even if we were to improve the size estimates (e.g. based on some average instruction size) the kernel examples would still be off because they switch sections back and forth, use asm macros and computed .fill directives and maybe further stuff. GCC will never be able to accurately calculate these sizes (without an built-in assembler which hopefully noone proposes). So, there is a case for extending the inline-asm facility to say "size is complicated here, assume this for inline decisions". > > Now, Richard suggested doing something like: > > > > 1) inline asm ("...") > > What would the semantics of this be? The size of the inline asm wouldn't be counted towards the inliner size limits (or be counted as "1"). > I don't like 2) either. But 1) looks interesting, depends what its > semantics would be? "Don't count this insn's size for inlining decisions", > maybe? TBH, I like the inline asm (...) suggestion most currently, but what if we want to add more attributes to asms? We could add further special keywords to the clobber list: asm ("...." : : : "cc,memory,inline"); sure, it might seem strange to "clobber" inline, but if we reinterpret the clobber list as arbitrary set of attributes for this asm, it'd be fine. > Another option is to just force inlining for those few functions where > GCC currently makes an inlining decision you don't like. Or are there > more than a few? I think the examples I saw from Boris were all indirect inlines: static inline void foo() { asm("large-looking-but-small-asm"); } static void bar1() { ... foo() ... } static void bar2() { ... foo() ... } void goo (void) { bar1(); } // bar1 should have been inlined So, while the immediate asm user was marked as always inline that in turn caused users of it to become non-inlined. I'm assuming the kernel guys did proper measurements that they _really_ get some non-trivial speed benefit by inlining bar1/bar2, but for some reasons (I didn't inquire) didn't want to mark them all as inline as well. Ciao, Michael.
next prev parent reply other threads:[~2018-10-07 15:53 UTC|newest] Thread overview: 186+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-10-03 21:30 [PATCH v9 00/10] x86: macrofying inline asm Nadav Amit 2018-10-03 21:30 ` Nadav Amit 2018-10-03 21:30 ` [PATCH v9 01/10] xtensa: defining LINKER_SCRIPT for the linker script Nadav Amit 2018-10-04 10:00 ` [tip:x86/build] kbuild/arch/xtensa: Define " tip-bot for Nadav Amit 2018-10-03 21:30 ` [PATCH v9 02/10] Makefile: Prepare for using macros for inline asm Nadav Amit 2018-10-03 21:30 ` Nadav Amit 2018-10-04 10:01 ` [tip:x86/build] kbuild/Makefile: Prepare for using macros in inline assembly code to work around asm() related GCC inlining bugs tip-bot for Nadav Amit 2018-11-06 18:57 ` [PATCH v9 02/10] Makefile: Prepare for using macros for inline asm Logan Gunthorpe 2018-11-06 19:18 ` Nadav Amit 2018-11-06 20:01 ` Logan Gunthorpe 2018-11-07 18:01 ` Nadav Amit 2018-11-07 18:53 ` Logan Gunthorpe 2018-11-07 18:56 ` Nadav Amit 2018-11-07 21:43 ` Logan Gunthorpe 2018-11-07 21:50 ` hpa 2018-11-08 6:18 ` Nadav Amit 2018-11-08 17:14 ` Logan Gunthorpe 2018-11-08 19:54 ` Nadav Amit 2018-11-08 20:00 ` Logan Gunthorpe 2018-11-08 20:18 ` Nadav Amit 2018-11-10 22:04 ` Nadav Amit 2018-11-13 4:56 ` Logan Gunthorpe 2018-10-03 21:30 ` [PATCH v9 03/10] x86: objtool: use asm macro for better compiler decisions Nadav Amit 2018-10-03 21:30 ` Nadav Amit 2018-10-04 10:02 ` [tip:x86/build] x86/objtool: Use asm macros to work around GCC inlining bugs tip-bot for Nadav Amit 2018-10-03 21:30 ` [PATCH v9 04/10] x86: refcount: prevent gcc distortions Nadav Amit 2018-10-04 7:57 ` Ingo Molnar 2018-10-04 8:33 ` Ingo Molnar 2018-10-04 8:40 ` hpa 2018-10-04 8:56 ` Ingo Molnar 2018-10-04 8:56 ` Nadav Amit 2018-10-04 9:02 ` hpa 2018-10-04 9:16 ` Ingo Molnar 2018-10-04 19:33 ` H. Peter Anvin 2018-10-04 20:05 ` Nadav Amit 2018-10-04 20:08 ` H. Peter Anvin 2018-10-04 20:29 ` Andy Lutomirski 2018-10-04 23:11 ` H. Peter Anvin 2018-10-06 1:40 ` Rasmus Villemoes 2018-10-04 9:12 ` Ingo Molnar 2018-10-04 9:17 ` hpa 2018-10-04 9:30 ` Nadav Amit 2018-10-04 9:45 ` Ingo Molnar 2018-10-04 10:23 ` Nadav Amit 2018-10-05 9:31 ` Ingo Molnar 2018-10-05 11:20 ` Borislav Petkov 2018-10-05 12:52 ` Ingo Molnar 2018-10-05 20:27 ` [PATCH 0/3] Macrofying inline asm rebased Nadav Amit 2018-10-05 20:27 ` [PATCH 1/3] x86/extable: Macrofy inline assembly code to work around GCC inlining bugs Nadav Amit 2018-10-06 14:42 ` [tip:x86/build] " tip-bot for Nadav Amit 2018-10-05 20:27 ` [PATCH 2/3] x86/cpufeature: " Nadav Amit 2018-10-06 14:43 ` [tip:x86/build] " tip-bot for Nadav Amit 2018-10-05 20:27 ` [PATCH 3/3] x86/jump-labels: " Nadav Amit 2018-10-06 14:44 ` [tip:x86/build] " tip-bot for Nadav Amit 2018-10-08 2:17 ` [PATCH v9 04/10] x86: refcount: prevent gcc distortions Nadav Amit 2018-10-04 8:40 ` Nadav Amit 2018-10-04 9:01 ` Ingo Molnar 2018-10-04 10:02 ` [tip:x86/build] x86/refcount: Work around GCC inlining bug tip-bot for Nadav Amit 2018-10-03 21:30 ` [PATCH v9 05/10] x86: alternatives: macrofy locks for better inlining Nadav Amit 2018-10-04 10:03 ` [tip:x86/build] x86/alternatives: Macrofy lock prefixes to work around GCC inlining bugs tip-bot for Nadav Amit 2018-10-03 21:30 ` [PATCH v9 06/10] x86: bug: prevent gcc distortions Nadav Amit 2018-10-04 10:03 ` [tip:x86/build] x86/bug: Macrofy the BUG table section handling, to work around GCC inlining bugs tip-bot for Nadav Amit 2018-10-03 21:30 ` [PATCH v9 07/10] x86: prevent inline distortion by paravirt ops Nadav Amit 2018-10-04 10:04 ` [tip:x86/build] x86/paravirt: Work around GCC inlining bugs when compiling " tip-bot for Nadav Amit 2018-10-03 21:30 ` [PATCH v9 08/10] x86: extable: use macros instead of inline assembly Nadav Amit 2018-10-03 21:30 ` [PATCH v9 09/10] x86: cpufeature: " Nadav Amit 2018-10-03 21:31 ` [PATCH v9 10/10] x86: jump-labels: " Nadav Amit 2018-10-07 9:18 ` PROPOSAL: Extend inline asm syntax with size spec Borislav Petkov 2018-10-07 9:18 ` Borislav Petkov 2018-10-07 9:18 ` Borislav Petkov 2018-10-07 13:22 ` Segher Boessenkool 2018-10-07 14:13 ` Borislav Petkov 2018-10-07 14:13 ` Borislav Petkov 2018-10-07 15:14 ` Segher Boessenkool 2018-10-07 15:14 ` Segher Boessenkool 2018-10-08 5:58 ` Ingo Molnar 2018-10-08 5:58 ` Ingo Molnar 2018-10-08 5:58 ` Ingo Molnar 2018-10-08 7:53 ` Segher Boessenkool 2018-10-08 7:53 ` Segher Boessenkool 2018-10-07 14:13 ` Borislav Petkov 2018-10-07 15:53 ` Michael Matz [this message] 2018-10-07 15:53 ` Michael Matz 2018-10-08 6:13 ` Ingo Molnar 2018-10-08 6:13 ` Ingo Molnar 2018-10-08 8:18 ` Segher Boessenkool 2018-10-08 8:18 ` Segher Boessenkool 2018-10-08 7:31 ` Segher Boessenkool 2018-10-08 7:31 ` Segher Boessenkool 2018-10-08 9:07 ` Richard Biener 2018-10-08 9:07 ` Richard Biener 2018-10-08 10:02 ` Segher Boessenkool 2018-10-08 10:02 ` Segher Boessenkool 2018-10-09 14:53 ` Segher Boessenkool 2018-10-09 14:53 ` Segher Boessenkool 2018-10-10 6:35 ` Ingo Molnar 2018-10-10 6:35 ` Ingo Molnar 2018-10-10 7:12 ` Richard Biener 2018-10-10 7:12 ` Richard Biener 2018-10-10 7:22 ` Ingo Molnar 2018-10-10 7:22 ` Ingo Molnar 2018-10-10 8:03 ` Segher Boessenkool 2018-10-10 8:03 ` Segher Boessenkool 2018-10-10 8:19 ` Borislav Petkov 2018-10-10 8:19 ` Borislav Petkov 2018-10-10 8:19 ` Borislav Petkov 2018-10-10 8:35 ` Richard Biener 2018-10-10 8:35 ` Richard Biener 2018-10-10 18:54 ` Segher Boessenkool 2018-10-10 18:54 ` Segher Boessenkool 2018-10-10 19:14 ` Borislav Petkov 2018-10-10 19:14 ` Borislav Petkov 2018-10-10 19:14 ` Borislav Petkov 2018-10-13 19:33 ` Borislav Petkov 2018-10-13 19:33 ` Borislav Petkov 2018-10-13 21:14 ` Alexander Monakov 2018-10-13 21:14 ` Alexander Monakov 2018-10-13 21:30 ` Borislav Petkov 2018-10-13 21:30 ` Borislav Petkov 2018-10-25 10:24 ` Borislav Petkov 2018-10-25 10:24 ` Borislav Petkov 2018-10-25 10:24 ` Borislav Petkov 2018-10-31 12:55 ` Peter Zijlstra 2018-10-31 12:55 ` Peter Zijlstra 2018-10-31 13:11 ` Peter Zijlstra 2018-10-31 13:11 ` Peter Zijlstra 2018-10-31 16:31 ` Segher Boessenkool 2018-10-31 16:31 ` Segher Boessenkool 2018-11-01 5:20 ` Joe Perches 2018-11-01 5:20 ` Joe Perches 2018-11-01 9:01 ` Peter Zijlstra 2018-11-01 9:01 ` Peter Zijlstra 2018-11-01 9:20 ` Joe Perches 2018-11-01 9:20 ` Joe Perches 2018-11-01 11:15 ` Peter Zijlstra 2018-11-01 11:15 ` Peter Zijlstra 2018-11-01 9:20 ` Joe Perches 2018-11-01 5:20 ` Joe Perches 2018-12-27 4:47 ` Masahiro Yamada 2018-12-27 4:47 ` Masahiro Yamada 2018-10-10 10:29 ` Richard Biener 2018-10-10 10:29 ` Richard Biener 2018-10-10 7:53 ` Segher Boessenkool 2018-10-10 7:53 ` Segher Boessenkool 2018-10-10 16:31 ` Nadav Amit 2018-10-10 16:31 ` Nadav Amit 2018-10-10 19:21 ` Segher Boessenkool 2018-10-10 19:21 ` Segher Boessenkool 2018-10-11 7:04 ` Richard Biener 2018-10-11 7:04 ` Richard Biener 2018-11-29 11:46 ` Masahiro Yamada 2018-11-29 11:46 ` Masahiro Yamada 2018-11-29 11:46 ` Masahiro Yamada 2018-11-29 12:25 ` Segher Boessenkool 2018-11-29 12:25 ` Segher Boessenkool 2018-11-30 9:06 ` Boris Petkov via Virtualization 2018-11-30 9:06 ` Boris Petkov 2018-11-30 9:06 ` Boris Petkov 2018-11-30 13:16 ` Segher Boessenkool 2018-11-30 13:16 ` Segher Boessenkool 2018-12-10 8:16 ` Masahiro Yamada 2018-12-10 8:16 ` Masahiro Yamada 2018-12-10 8:16 ` Masahiro Yamada 2018-11-29 13:07 ` Borislav Petkov 2018-11-29 13:07 ` Borislav Petkov via Virtualization 2018-11-29 13:09 ` Richard Biener 2018-11-29 13:09 ` Richard Biener 2018-11-29 13:16 ` Borislav Petkov via Virtualization 2018-11-29 13:16 ` Borislav Petkov 2018-11-29 13:16 ` Borislav Petkov 2018-11-29 13:24 ` Richard Biener 2018-11-29 13:24 ` Richard Biener 2018-10-08 16:24 ` David Laight 2018-10-08 16:24 ` David Laight 2018-10-08 16:24 ` David Laight 2018-10-07 16:09 ` Nadav Amit 2018-10-07 16:09 ` Nadav Amit 2018-10-07 16:13 ` [RESEND] " Nadav Amit 2018-10-07 16:46 ` Richard Biener 2018-10-07 16:46 ` Richard Biener 2018-10-07 19:06 ` Nadav Amit 2018-10-07 19:06 ` Nadav Amit 2018-10-07 19:52 ` Jeff Law 2018-10-07 19:52 ` Jeff Law 2018-10-08 7:46 ` Richard Biener 2018-10-08 7:46 ` Richard Biener
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