> -----Original Message----- > From: Jeff Law [mailto:l...@redhat.com] > Sent: Tuesday, September 27, 2011 12:43 AM > To: Richard Guenther > Cc: Jiangning Liu; gcc@gcc.gnu.org > Subject: Re: A case that PRE optimization hurts performance > > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On 09/26/11 05:00, Richard Guenther wrote: > > On Mon, Sep 26, 2011 at 9:39 AM, Jiangning Liu > > <jiangning....@arm.com> wrote: > >>>> * Without PRE, > >>>> > >>>> Path1: movl $2, %eax cmpl $1, %eax je .L3 > >>>> > >>>> Path2: movb $3, %al cmpl $1, %eax je .L3 > >>>> > >>>> Path3: cmpl $1, %eax jne .L14 > >>>> > >>>> * With PRE, > >>>> > >>>> Path1: movl $1, %ebx movl $2, %eax testb %bl, %bl je > >>>> .L3 > >>>> > >>>> Path2: movl $1, %ebx movb $3, %al testb %bl, %bl je > >>>> .L3 > >>>> > >>>> Path3: cmpl $1, %eax setne %bl testb %bl, %bl jne > >>>> .L14 > >>>> > >>>> Do you have any more thoughts? > >>> > >>> It seems to me that with PRE all the testb %bl, %bl should be > >>> evaluated at compile-time considering the preceeding movl $1, > >>> %ebx. Am I missing something? > >>> > >> > >> Yes. Can this be done by PRE or any other optimizations in middle > >> end? > > > > Hm, the paths as you quote them are obfuscated by missed > > jump-threading. On the tree level we have > > > > # s_2 = PHI <2(5), 3(4), 2(6), s_25(7)> # prephitmp.6_1 = PHI > > <1(5), 1(4), 1(6), prephitmp.6_3(7)> <L10>: t_14 = t_24 + 1; > > D.2729_6 = MEM[base: t_14, offset: 0B]; D.2732_7 = D.2729_6 != 0; > > D.2734_9 = prephitmp.6_1 & D.2732_7; if (D.2734_9 != 0) > > > > where we could thread the cases with prephitmp.6_1 == 1, > > ultimately removing the & and forwarding the D.2729_6 != 0 test. > > Which would of course cause some code duplication. > > > > Jeff, you recently looked at tree jump-threading, can you see if > > we can improve things on this particular testcase? > There's nothing threading can do here because it doesn't know anything > about the value MEM[t14]. >
Jeff, Could you please explain more about this? What information does jump threading want to know on MEM[t14]? Do you mean it's hard to duplicate that basic block due to some reasons? Thanks, -Jiangning > > Jeff > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.11 (GNU/Linux) > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ > > iQEcBAEBAgAGBQJOgKuLAAoJEBRtltQi2kC75aIH/iikuOQXrMrQJFbQw0COXznB > OGq8iXdGwTJGH13vxdItTE0upJp7RgUVLzuhdqj1elTLHv/ujYygMsQRNGKcc8tb > GMLECmWDhZqQTFXcTJCgJNZiv7MH1PNELXSdIkkSnxY+pwyn9AX5D3+HcTSjGU6B > 51AdUNVph/VSaVboAgcrFpu9S0pX9HVTqFy4JI83Lh613zDVSmPo14DDy7vjBvE9 > 2Srlvlw0srYup97bGmRqN8wT4ZLLlyYSB2rjEFc6jmgXVncxiteQYIUZpy0lcC0M > q3j80aXjZ57/iWyAbqDr1jI5tbVKDBkRa9LL1jvn9534adiG4GrnSMPhoog0ibA= > =azr5 > -----END PGP SIGNATURE-----
