[LLVMdev] [X86] DAG Combine - VSELECT

[Demikhovsky, Elena]

Can this optimization be moved to the lowering phase? LowerVSELECT() ?

-           Elena

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Eli Friedman
Sent: Tuesday, August 20, 2013 03:56
To: Juergen Ributzka
Cc: Benjamin Kramer; LLVM Developers Mailing List
Subject: Re: [LLVMdev] [X86] DAG Combine - VSELECT

On Mon, Aug 19, 2013 at 4:17 PM, Juergen Ributzka <juergen at apple.com<mailto:juergen at apple.com>> wrote:
I see. We still can use that shortcut to catch the simple case after type legalization, but we could also do a more elaborate type check before type legalization to enable it?

If you're going to write the code to check the types anyway, it's probably clearer to remove the isBeforeLegalize check.

-Eli

On Aug 19, 2013, at 4:13 PM, Eli Friedman <eli.friedman at gmail.com<mailto:eli.friedman at gmail.com>> wrote:


On Mon, Aug 19, 2013 at 3:34 PM, Juergen Ributzka <juergen at apple.com<mailto:juergen at apple.com>> wrote:
Hi @ll,

I am wondering about the use of !isBeforeLegalize in PerformSELECTCombine in the X86 backend. This defers all VSELECT related DAG combines until after the Legalizer has run. If the IR has already only legal types the second round of DAG combines is skipped and no VSELECT specified optimizations are performed at all.

Is there a reason we don't run the X86 DAG combiner before Type Legalization?

 I think the code in question is using isBeforeLegalize as a shortcut for "are all the operand and result types legal".

-Eli


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