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Re: Enable EBX for x86 in 32bits PIC code

From: Vladimir Makarov <vmakarov at redhat dot com>
To: Ilya Enkovich <enkovich dot gnu at gmail dot com>
Cc: gcc at gnu dot org, gcc-patches <gcc-patches at gcc dot gnu dot org>, Evgeny Stupachenko <evstupac at gmail dot com>, Richard Biener <richard dot guenther at gmail dot com>, Uros Bizjak <ubizjak at gmail dot com>, Jeff Law <law at redhat dot com>
Date: Wed, 03 Sep 2014 16:19:12 -0400
Subject: Re: Enable EBX for x86 in 32bits PIC code
Authentication-results: sourceware.org; auth=none
References: <CAOvf_xxsQ_oYGqNAVQ1+BW+CuD3mzebZ2xma0jpF=WfyZMCRCA at mail dot gmail dot com> <CAFiYyc1mFtTezkTJORmJJq+yht=qPSwiN7KDn19+bSuSdaqvMQ at mail dot gmail dot com> <CAOvf_xyeVeg2oB9Xxz8RMEQ6gyfJY5whd9s4ygoAAEaMU9efnA at mail dot gmail dot com> <20140707114750 dot GB31640 at tucnak dot redhat dot com> <CAMbmDYZV_fx0jxmKHhLsC2pJ7pDzuu6toEAH72izOdpq6KGyfg at mail dot gmail dot com> <20140822121151 dot GA60032 at msticlxl57 dot ims dot intel dot com> <53FB5184 dot 3030500 at redhat dot com> <CAMbmDYbP77pmkqpJD8cnXbe7_8aRMnYF-tnfKRubULn8-aDCdw at mail dot gmail dot com> <CAMbmDYacBWjKtCYPB0A2m=fkUTk_Wt5D6f2aEkH7C3paWaR7ag at mail dot gmail dot com> <53FCA6F5 dot 7020405 at redhat dot com> <20140826213712 dot GA39451 at msticlxl57 dot ims dot intel dot com> <53FE3D46 dot 8050904 at redhat dot com> <CAMbmDYZxt7wceND52mb9rM+poTNpjFtLyj2WT5qatbjMdLWmbg at mail dot gmail dot com> <CAMbmDYb9E6-DpCd8cx+X-4d3oDkLMTo63ibojAY+Lsf9L-UNRA at mail dot gmail dot com>

On 2014-08-29 2:47 AM, Ilya Enkovich wrote:

Seems your patch doesn't cover all cases.  Attached is a modified
patch (with your changes included) and a test where double constant is
wrongly rematerialized.  I also see in ira dump that there is still a
copy of PIC reg created:

Initialization of original PIC reg:
(insn 23 22 24 2 (set (reg:SI 127)
         (reg:SI 3 bx)) test.cc:42 90 {*movsi_internal}
      (expr_list:REG_DEAD (reg:SI 3 bx)
         (nil)))
...
Copy is created:
(insn 135 37 25 3 (set (reg:SI 138 [127])
         (reg:SI 127)) 90 {*movsi_internal}
      (expr_list:REG_DEAD (reg:SI 127)
         (nil)))
...
Copy is used:
(insn 119 25 122 3 (set (reg:DF 134)
         (mem/u/c:DF (plus:SI (reg:SI 138 [127])
                 (const:SI (unspec:SI [
                             (symbol_ref/u:SI ("*.LC0") [flags 0x2])
                         ] UNSPEC_GOTOFF))) [5  S8 A64])) 128 {*movdf_internal}
      (expr_list:REG_EQUIV (const_double:DF
2.9999999999999997371893933895137251965934410691261292e-4
[0x0.9d495182a99308p-11])
         (nil)))


The copy is created by a newer IRA optimization for function prologues.

The patch in the attachment should solve the problem. I also added thecode to prevent spilling the pic pseudo in LRA which could happen beforetheoretically.

After reload we have new usage of r127 which is allocated to ecx which
actually does not have any definition in this function at all.

(insn 151 42 44 4 (set (reg:SI 0 ax [147])
         (plus:SI (reg:SI 2 cx [127])
             (const:SI (unspec:SI [
                         (symbol_ref/u:SI ("*.LC0") [flags 0x2])
                     ] UNSPEC_GOTOFF)))) test.cc:44 213 {*leasi}
      (expr_list:REG_EQUAL (symbol_ref/u:SI ("*.LC0") [flags 0x2])
         (nil)))
(insn 44 151 45 4 (set (reg:DF 21 xmm0 [orig:129 D.2450 ] [129])
         (mult:DF (reg:DF 21 xmm0 [orig:128 D.2450 ] [128])
             (mem/u/c:DF (reg:SI 0 ax [147]) [5  S8 A64]))) test.cc:44
790 {*fop_df_comm_sse}
      (expr_list:REG_EQUAL (mult:DF (reg:DF 21 xmm0 [orig:128 D.2450 ] [128])
             (const_double:DF
2.9999999999999997371893933895137251965934410691261292e-4
[0x0.9d495182a99308p-11]))
         (nil)))

Compilation string: g++ -m32 -O2 -mfpmath=sse -fPIE -S test.cc

Attachment: z2
Description: Text document

Follow-Ups:
- Re: Enable EBX for x86 in 32bits PIC code
  - From: Ilya Enkovich

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