[llvm-dev] Question about pattern matching process

[Craig Topper via llvm-dev]

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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