On Mon, Sep 26, 2011 at 6:42 PM, Jeff Law <l...@redhat.com> wrote: > -----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].
It knows something about prephitmp.6_1 and thus could simplify D.2734_9 = prephitmp_6.1 & D.2732_7; to D.2734_9 = D.2732_7; But admittedly I have no idea if DOM tries to simplify things other than comparisons within the jump threading machinery ... (the above block basically ends in if (D.2729_6 != 0 && prephitmp_6.1), so I'd guess it's worth to simplify the (complex) conditional expression). Richard. > > 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----- >
