[llvm-dev] Question about pattern matching process

Sat Feb 9 15:27:12 PST 2019

Patterns are first sorted by their "complexity" which is mostly just the
number of nodes the pattern matches. More complex patterns get the hightest
priority. In the even of a tie in complexity, the earlier pattern in the
.td file gets priority. You can artificially bump the priority of a pattern
by adding something like "let AddedComplexity = 1" to the pattern. The
AddedComplexity can also take a negative value to lower its priority.

~Craig

On Sat, Feb 9, 2019 at 3:18 PM Josh Sharp <mm92126 at hotmail.com> wrote:

> Thanks, Craig.
>
> Unfortunately I can't. But I tried moving the definition of MOVRR to the
> bottom of the XXXInstrInfo.td file and it worked. Now the correct
> instruction comes first in array in XXXGenDAGISel.inc and it gets properly
> selected during ISEL.
> Is this the right way to solve this kind of problem?
>
>
> Josh
> ------------------------------
> *From:* Craig Topper <craig.topper at gmail.com>
> *Sent:* Saturday, February 9, 2019 2:06 PM
> *To:* Josh Sharp
> *Cc:* via llvm-dev
> *Subject:* Re: [llvm-dev] Question about pattern matching process
>
> I don't think it really went backwards. The "failed at index" uses a
> cached index not the current index the table is looking at. The index
> points to the last index that was processed by the opcode decoding switch.
> I suspect in your case 3259 is the start of an Opc_Scope. An Opc_Scope has
> multiple children. There's an optimization in Opc_Scope handler to walk
> each child and see if its even possibly feasible. This avoids pushing the
> scope stack only to immediately pop it again. This walking of the children
> and rejecting them prints the "Skipped Scope Entry" messages.
> Unfortunately, there's no message to indicate when the scope is actually
> pushed. I suspect in your case all of the children of the Opc_Scope at 3259
> were rejected. This would be messages at 3261, 3273, and 3285. So the
> entire scope fails and prints the "Match failed at index 3259" since that's
> where the Opc_Scope it was processing started. 3305 should be the end
> marker for the scope. Then the stack pops which prints the continuing at
> 3306 message.
>
> Are you able to share your XXXGenDAGISel.inc?
>
> ~Craig
>
>
> On Sat, Feb 9, 2019 at 12:35 PM Josh Sharp via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi,
> I'd like to understand the order in which patterns are searched during
> ISEL. In the example below, indices are searched in ascending order from
> 808 to 3305, then it goes back to 3259 and eventually it matches the wrong
> instruction. Why did go back from 3305 to 3259? In my XXXGenDAGISel.inc, I
> can see that the correct instruction is at index 3420 but it never got
> there.
>
> ISEL: Starting pattern match
>   Skipped scope entry (due to false predicate) at index 3, continuing at
> 808
>   Skipped scope entry (due to false predicate) at index 809, continuing at
> 865
>   Skipped scope entry (due to false predicate) at index 867, continuing at
> 1091
>   Skipped scope entry (due to false predicate) at index 1093, continuing
> at 1578
>   Skipped scope entry (due to false predicate) at index 1579, continuing
> at 1605
>   Skipped scope entry (due to false predicate) at index 1606, continuing
> at 1668
>   Skipped scope entry (due to false predicate) at index 1670, continuing
> at 2369
>   Skipped scope entry (due to false predicate) at index 2370, continuing
> at 2397
>   Skipped scope entry (due to false predicate) at index 2398, continuing
> at 2451
>   Skipped scope entry (due to false predicate) at index 2452, continuing
> at 2562
>   Skipped scope entry (due to false predicate) at index 2563, continuing
> at 2641
>   Skipped scope entry (due to false predicate) at index 2642, continuing
> at 2669
>   Skipped scope entry (due to false predicate) at index 2670, continuing
> at 2696
>   Skipped scope entry (due to false predicate) at index 2697, continuing
> at 2779
>   Skipped scope entry (due to false predicate) at index 2780, continuing
> at 2835
>   Skipped scope entry (due to false predicate) at index 2836, continuing
> at 2891
>   Skipped scope entry (due to false predicate) at index 2892, continuing
> at 2947
>   Skipped scope entry (due to false predicate) at index 2948, continuing
> at 2999
>   Skipped scope entry (due to false predicate) at index 3000, continuing
> at 3032
>   Skipped scope entry (due to false predicate) at index 3033, continuing
> at 3053
>   Skipped scope entry (due to false predicate) at index 3054, continuing
> at 3085
>   Skipped scope entry (due to false predicate) at index 3086, continuing
> at 3114
>   Skipped scope entry (due to false predicate) at index 3115, continuing
> at 3171
>   Skipped scope entry (due to false predicate) at index 3172, continuing
> at 3201
>   Skipped scope entry (due to false predicate) at index 3202, continuing
> at 3253
>   Skipped scope entry (due to false predicate) at index 3261, continuing
> at 3272
>   Skipped scope entry (due to false predicate) at index 3273, continuing
> at 3284
>   Skipped scope entry (due to false predicate) at index 3285, continuing
> at 3305
>   Match failed at index 3259
>   Continuing at 3306
>   Match failed at index 3307
>   Continuing at 3327
>   Match failed at index 3328
>   Continuing at 3361
>   Morphed node: t17: i32 = MOVRR t17
> ISEL: Match complete!
>
>
> Thanks.
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