From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 3136 invoked by alias); 1 May 2015 00:31:36 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 3124 invoked by uid 89); 1 May 2015 00:31:35 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.3 required=5.0 tests=AWL,BAYES_00,KAM_ASCII_DIVIDERS,KAM_STOCKGEN,RCVD_IN_DNSWL_LOW,SPF_PASS,T_RP_MATCHES_RCVD autolearn=no version=3.3.2 X-HELO: mail-vn0-f45.google.com Received: from mail-vn0-f45.google.com (HELO mail-vn0-f45.google.com) (209.85.216.45) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES128-GCM-SHA256 encrypted) ESMTPS; Fri, 01 May 2015 00:31:33 +0000 Received: by vnbg1 with SMTP id g1so9195045vnb.2 for ; Thu, 30 Apr 2015 17:31:31 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:date:message-id:subject:from:to :content-type; bh=ueYjkdy8Jt2o0eBWXFLVpDCibAVdvefgDPvo40ALkao=; b=L1KYpQk7IZ5Ta9rPPqEc6oy27QDoJG//kh+156WsIMWHOB++9CGfZTQgaEq9a7bCPC b3GkWqUNYC32K4/+0Ve5XA5hWRAnDoZwh2jYiDxvO1RzszTdHNcGRKnI2Lb3H3h9v4CK 5FcYqONxFe+Xgm3WZUf4xPEj0x1a2X/9+GLoKZCsSfb6ynjKxXuqHW8HurpP0HwCXo/O TyyebGQXS2kDfDW5Cx5dX3kTACARLCsVq8tHnwJY9LC8Cl2ncTlddEwLKPQr9cXjDXmz 1wL4f466zX834GWvh17j+mpgc62RWyWzt8QjpfmWJl6LW7HUvh5H46ggpjA7K4NWMYxO kJ1g== X-Gm-Message-State: ALoCoQlh9EVaaG0TkLqykxZ4qID0x/J2O1D2AeLB/P3iFCVmWxbJ/8Pw9eqA/NoNxQbHR39eWPe0 MIME-Version: 1.0 X-Received: by 10.52.240.198 with SMTP id wc6mr11901270vdc.34.1430440291039; Thu, 30 Apr 2015 17:31:31 -0700 (PDT) Received: by 10.52.122.52 with HTTP; Thu, 30 Apr 2015 17:31:30 -0700 (PDT) Date: Fri, 01 May 2015 00:31:00 -0000 Message-ID: Subject: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt= From: Sriraman Tallam To: GCC Patches , "H.J. Lu" , David Li Content-Type: multipart/mixed; boundary=20cf307810b26c14520514fa55d5 X-IsSubscribed: yes X-SW-Source: 2015-05/txt/msg00001.txt.bz2 --20cf307810b26c14520514fa55d5 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Content-length: 3786 Hi, We noticed that one of our benchmarks sped-up by ~1% when we eliminated PLT stubs for some of the hot external library functions like memcmp, pow. The win was from better icache and itlb performance. The main reason was that the PLT stubs had no spatial locality with the call-sites. I have started looking at ways to tell the compiler to eliminate PLT stubs (in-effect inline them) for specified external functions, for x86_64. I have a proposal and a patch and I would like to hear what you think. Here is a summary of what is happening currently. A call to an external function is direct but calls into the PLT stub which then jumps indirectly to the GOT entry. If I could replace the direct call to the PLT stub with an indirect call to a GOT entry which will hold the address of the external function, I have gotten rid of the PLT stub. Here is an example: foo.cc =3D=3D=3D=3D=3D extern int foo (); // Truly external library function, defined in a shared library. int main() { foo(); ... } Currently, this is what is happening. foo.s looks like this: main: ..... callq _Z3foov but the linker replaces this to call the PLT stub of foo instead. Function main calls the plt stub directly: 0000000000400766
: =E2=80=A6. 40076a: e8 71 fe ff ff callq 4005e0 <_Z3foov@plt> and the PLT stub does this: 00000000004005e0 <_Z3foov@plt>: 4005e0: jmpq *0x15d2(%rip) # 401bb8 <_GLOBAL_OFFSET_TABLE_+0x28> 4005e6: pushq $0x2 4005eb: jmpq 4005b0 <_init+0x28> The GOT entry at address 0x401bb8 contains the address of foo which will be lazily bound. What my proposal plans does is to change foo.s to look like this: callq *_Z3foov@GOTPCREL(%rip) which is indirectly calling foo via a GOT entry that contains the address of foo. The address in the GOT entry is fixed up at load time and the linker creates only one GOT entry per function irrespective of the number of callers. a.out now looks like this: 0000000000400746
: ... 40074a: ff 15 20 14 00 00 callq *0x1420(%rip) # 401b70 <_DYNAMIC+0x1e8> ... Function main indirectly calls foo using the contents at location 0x401b70 which is actually a GOT entry containing the address of foo. Notice that we have in effect inlined the PLT stub. This comes with caveats. This cannot be generally done for all functions marked extern as it is impossible for the compiler to say if a function is "truly extern" (defined in a shared library). If a function is not truly extern(ends up defined in the final executable), then calling it indirectly is a performance penalty as it could have been a direct call. Further, the newly created GOT entries are fixed up at start-up and do not get lazily bound. Given this, I propose adding a new option called -fno-plt=3D to the compiler. This tells the compiler that we know that the function is truly extern and we want the indirect call only for these call-sites. I have attached a patch that adds -fno-plt=3D to GCC. Any number of "-fno-plt=3D" can be specified and all call-sites corresponding to these named functions will be done indirectly using the mechanism described above without the use of a PLT stub. Alternatively, we can do this entirely in the linker. We can introduce a new relocation type to tell the linker to convert all direct calls to truly extern functions into indirect calls via GOT entries. The GCC patch just seems simpler. Also, we could link statically but we do not want that or we could copy the specific external functions into our executable. This might work for executable A but a different set of external functions might be hot for executable B. We want a more general solution. Please let me know what you think. Thanks Sri --20cf307810b26c14520514fa55d5 Content-Type: text/plain; charset=US-ASCII; name="avoid_plt_patch.txt" Content-Disposition: attachment; filename="avoid_plt_patch.txt" Content-Transfer-Encoding: base64 X-Attachment-Id: f_i94tl63r0 Content-length: 5547 CSogY29tbW9uLm9wdCAoLWZuby1wbHQ9KTogTmV3IG9wdGlvbi4KCSogY29u ZmlnL2kzODYvaTM4Ni5jIChhdm9pZF9wbHRfdG9fY2FsbCk6IE5ldyBmdW5j dGlvbi4KCShpeDg2X291dHB1dF9jYWxsX2luc24pOiAgQ2hlY2sgaWYgUExU IG5lZWRzIHRvIGJlIGF2b2lkZWQKCWFuZCBjYWxsIG9yIGp1bXAgaW5kaXJl Y3RseSBpZiB0cnVlLgoJKiBvcHRzLWdsb2JhbC5jIChodGFiX3N0cl9lcSk6 IE5ldyBmdW5jdGlvbi4KCShhdm9pZF9wbHRfZm5zeW1ib2xfbmFtZXNfdGFi KTogTmV3IGh0YWIuCgkoaGFuZGxlX2NvbW1vbl9kZWZlcnJlZF9vcHRpb25z KTogSGFuZGxlIC1mbm8tcGx0PQoKSW5kZXg6IGNvbW1vbi5vcHQKPT09PT09 PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09 PT09PT09PT09PT09PT09PQotLS0gY29tbW9uLm9wdAkocmV2aXNpb24gMjIy NjQxKQorKysgY29tbW9uLm9wdAkod29ya2luZyBjb3B5KQpAQCAtMTA4Nyw2 ICsxMDg3LDExIEBAIGZkYmctY250PQogQ29tbW9uIFJlamVjdE5lZ2F0aXZl 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