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Subject: 4.4.2 PowerPC Optimisation
Date: Tue, 22 Dec 2009 17:36:00 -0000
Message-ID: <80BF44D5D0EFF249A5522B3EFE78C0C5017EF788@2000server.aerltd.com>
From: "Mark Colby" <markc@liferacing.com>
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Hi,

I am trying to migrate my C/C++ cross-compiling (cygwin->powerpc) from
2.9.5 to 4.4.2. Specifically I have been successfully using 2.9.5 for
years and have just built 4.4.2 (no libraries) as I want to compile for
newer cores with isel, sse etc. I appear to have a fully working
compiler (produces a few hundred lines of code that runs correctly in my
target hardware) but the code produced is oddly inefficient despite -O3.
Under 2.9.5 this:

  typedef union
  {
    U32 R;
    struct
    {
      U32 WP:2;
      U32 WRC:2;
      U32 WIE:1;
      U32 DIE:1;
      U32 FP:2;
      U32 FIE:1;
      U32 ARE:1;
      U32 :1;
      U32 WPEXT:4;
      U32 FPEXT:4;
      U32 :13;
    } B;
  } SPR_TCRVAL_t;

  ..
=20=20
  {
    SPR_TCRVAL_t tcr;
    tcr.R=3D0;
    tcr.B.WRC=3D2;
    tcr.B.WPEXT=3D10;
    tcr.B.ARE=3D1;
    <asm macro to assign tcr to SPR 340>
  }

produces a nice simple:

  lis 0,0x2054
  mtspr 340,0

Yet under 4.4.2 with all optimisations apparently enabled (constant
optimisation, function inlining etc all visible in the assembly output)
I get:

  li 9,2
  li 0,0
  rlwimi 0,9,28,2,3
  li 9,10
  rlwimi 0,9,17,11,14
  li 9,1
  rlwimi 0,9,22,9,9
  mtspr 340,0

!!

Has anyone seen this sort of thing before? Is there anything obviously
dumb in what I am trying to do? Any pointers gratefully received!

Thanks,

Mark


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