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>In none of the TargetInstructionInfo hooks we obtain both the basic block and
<div>>branch conditions to detect this break of data dependency, although I imagine</div>
<div>>the if-converter should be the one doing so.</div>
<div>></div>
<div>>At best, isProfitableToIfCvt should also receive the branch conditions, so it</div>
<div>>could detect this case.</div>
<div><br>
</div>
<div>Which if-converter pass are you using? There is the "late" IfConverter pass and</div>
<div>the "early" EarlyIfPredicator pass. In either case, the legality checks are not</div>
<div>performed by isProfitableToIfCvt.</div>
<div><br>
</div>
<div>Off the top of my head, I'd guess that if you are using the IfConverter, you</div>
<div>might double-check your implementation of TII::ClobbersPredicate or trace</div>
<div>through its callers on your example. If you are using EarlyIfPredicator, you</div>
<div><span>might check SSAIfConv::canPredicateInstrs (and its helper SSAIfConv::InstrDependenciesAllowIfConv).</span></div>
<div><br>
</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Diogo Sampaio via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Sent:</b> Friday, September 24, 2021 10:53 AM<br>
<b>To:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [llvm-dev] If-converter breaking condition data dependency</font>
<div> </div>
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<br>
Hi,<br>
I'm trying to use if-converter to generate conditional load instructions. However I'm acing a data-dependency issue such as this one:<br>
The original code would output as this:<br>
<br>
compw.eq $r0 = $r1, $r0 <= condition is stored in r0<br>
;;<br>
cb.even $r0 ? .LBB72_9 <= branch if r0 is even<br>
;;<br>
ld $r0 = 16[$r20] <= only loads if r0 is odd<br>
;;<br>
ld $r20 = 8[$r0] <= only loads if r0 is odd<br>
;;<br>
.LBB72_9:<br>
<br>
<br>
if-converter would change it to this:<br>
compw.eq $r0 = $r1, $r0 <= same condition in same register<br>
;;<br>
ld.odd $r0 ? $r0 = 16[$r20] <= ko. The condition is overwritten.<br>
;;<br>
ld.odd $r0 ? $r20 = 8[$r0] <= ko. Load occurs depending on the new loaded value.<br>
<br>
In none of the TargetInstructionInfo hooks we obtain both the basic block and branch conditions to detect this break of data dependency, although I imagine the if-converter should be the one doing so.<br>
At best, isProfitableToIfCvt should also receive the branch conditions, so it could detect this case.<br>
<br>
Am I missing something?<br>
Did anyone else face such issue?<br>
Btw, we're still using llvm 12.0.1.<br>
<br>
Thanks in advance.<br>
<br>
Diogo Sampaio<br>
Senior Compiler Engineer • Kalray<br>
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