-----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].
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