[RFC][patch 3/3] Add support for variant scheduling classes in llvm-mca.

[andreadb]

Summary

This patch is the third of a sequence of three patches related to LLVM-dev RFC "MC support for variant scheduling classes". http://lists.llvm.org/pipermail/llvm-dev/2018-May/123181.html

This patch requires D47077 to be applied first.

The main goal of this patch is to teach llvm-mca how to solve variant scheduling classes.
This patch does that, plus it adds a variant scheduling class to the BtVer2 scheduling model to identify so-called zero-idioms (data dependency breaking instructions that are known to produce zero, and that are optimized out at register renaming stage).

Without the BtVer2 change, this patch would not have had any tests.
This patch is effectively the union of two changes:

1. the llvm-mca change that enables the resolution of variant scheduling classes, and
2. the change to the BtVer2 scheduling model.

Point 2. (partially) fixes PR36671.
Point 1. fixes PR36672.

@RKSimon and @craig.topper , the new scheduling predicate for the XOR zero-idiom is quite simple. In future, we could move predicates that are valid for multiple processor models into a common .td file.
For now, I keep that predicate check into the BtVer2 model.

Please let me know if okay to commit.

Thanks
-Andrea

[RKSimon]

lib/Target/X86/X86ScheduleBtVer2.td

553

Add a TODO saying this may go into X86Schedule.td in the future?

578

Reference Agner's microarchitecture doc as well - he explicity says this works for Jaguar - AMD 16h SOG unfortunately misses it and just having a reference to a different cpu is confusing.

test/CodeGen/X86/sse-schedule.ll

6084

This is unfortunate - ideally we'd keep [1:0.50] here and zero idioms would get [0:0.50]

test/tools/llvm-mca/X86/BtVer2/zero-idioms.s

33

Should the RThroughput still be limited to by issue width?

tools/llvm-mca/InstrBuilder.cpp

385

Drop braces - also is it worth pulling out SM.getProcessorID()? It might get this down to a single line for tidiness.

[andreadb]

lib/Target/X86/X86ScheduleBtVer2.td

553

Will do.

578

Okay. I will add that reference.

test/CodeGen/X86/sse-schedule.ll

6084

I should have mentioned this in the summary. The existing functionality used to obtain the latency from a MCInst doesn't know how to resolve variant scheduling classes. So, it accepts a machine opcode ID, and it bails out if the scheduling class identifier from the opcode descriptor refers to a variant class.
The idea is to fix it. However, it would be a follow-up patch. I can add a FIXME comment there.

test/tools/llvm-mca/X86/BtVer2/zero-idioms.s

33

It doesn't take into account the issue width. In this case, the MCSchedInfo method returned an invalid RThroughput (an Optional<double>).

On the other hand, the Block RThroughput correctly reports 3.0 for the sequence of 6 instructions (for a total of 6 uOps).

tools/llvm-mca/InstrBuilder.cpp

385

Okay. I will fix it.

[andreadb]

Patch updated.

Addressed review comments.

[courbet]

lib/Target/X86/X86ScheduleBtVer2.td

572

I think you should also set NumMicroOps to 0 here.

[andreadb]

lib/Target/X86/X86ScheduleBtVer2.td

572

Here, the number of opcodes is what gets subtracted to the IssueWidth budget for the current cycle.
On Jaguar, that quantity matches what the docs define as COP (complex opcode). Think of it as a container of uOps. An SSE xor is always decoded into a single COP. That COP is then sent to the RCU (and it takes one slot in the reorder buffer), and eliminated at register renaming stage.
Basically, although it is zero-latency, it still has to be retired. It consumes one slot in the ROB, and one slot in the dispatch group (subtracted to what llvm-mca calls "DispatchWidth" - the equivalent of the IssueWidth in the scheduling model).

[RKSimon]

lib/Target/X86/X86ScheduleBtVer2.td

563

Just to be clear, can this work for GPR zero-idioms as well (XOR32rr/XOR64rr etc)?

[andreadb]

lib/Target/X86/X86ScheduleBtVer2.td

563

Yes. JZeroIdiomPredicate can be used to describe XOR32rr and XOR64rr zero-idioms too.

You would still need a fall-back strategy for when the XOR is not a zero idiom.
That means, you would need to add the following case to JWriteZeroIdiom:

SchedVar<MCSchedPredicate<JIntXOR>, [WriteALU]>

where JIntXOR is defined as a CheckOpcode<[XOR32rr, XOR64rr]>.

[andreadb]

Patch updated.

This patch is now dependent on D47536.
D47536 avoids the loss of test coverage in -print-schedule tests.

[andreadb]

Ping.

[andreadb]

Patch updated.

Simplified the scheduling predicates as suggested off-line by SimonP.

[andreadb]

Patch updated (sorry for the spam).

This time, with an updated/extended test case.

[RKSimon]

lib/Target/X86/X86ScheduleBtVer2.td

558

This should probably be moved to X86Schedule.td straightaway - I see no benefit keeping this here as zeroidioms are something we're going to want in all x86 scheduler models.

Rename either to X86ZeroIdiomPredicate or X86SameRegOps01 (or something like that.....).

[andreadb]

Address review comments.

[andreadb]

Patch updated. This time with the correct diff...

[RKSimon]

CodeGen/X86/sse-schedule.ll

6228 ↗

(On Diff #149712)

These are unfortunate - please can you raise an upstream bugzilla about throughput printing defaulting to issues width.

llvm-mca/InstrBuilder.cpp

388 ↗

(On Diff #149712)

So !SchedClassID can only occur here due to variant resolution failing?

[andreadb]

CodeGen/X86/sse-schedule.ll

6228 ↗

(On Diff #149712)

Makes sense. I will raise a bug for it.

llvm-mca/InstrBuilder.cpp

388 ↗

(On Diff #149712)

Good point.
Strictly speaking, the scheduling class ID associated with MCI *should* always be valid. But I cannot guarantee it.. I will add an if-stmt to guard against invalid classes.

[andreadb]

Address Simon's review comment.

Raised https://bugs.llvm.org/show_bug.cgi?id=37678 for the -print-schedule issue with the throughput computation of zero-latency instructions.

[andreadb]

Patch updated.

[RKSimon]

You've lost the diff context, but LGTM - thanks.

lib/Target/X86/X86Schedule.td

 def SchedWriteFSqrtSizes
  : X86SchedWriteSizes<SchedWriteFSqrt, SchedWriteFSqrt64>;
 def SchedWriteFLogicSizes
  : X86SchedWriteSizes<SchedWriteFLogic, SchedWriteFLogic>;
 def SchedWriteFShuffleSizes
  : X86SchedWriteSizes<SchedWriteFShuffle, SchedWriteFShuffle>;
 
 //===----------------------------------------------------------------------===//
+//  Common MCInstPredicate definitions used by variant scheduling classes.
+
+def ZeroIdiomPredicate : CheckSameRegOperand<1, 2>;
+
+
+//===----------------------------------------------------------------------===//
 // Generic Processor Scheduler Models.
 
 // IssueWidth is analogous to the number of decode units. Core and its
 // descendents, including Nehalem and SandyBridge have 4 decoders.
 // Resources beyond the decoder operate on micro-ops and are bufferred
 // so adjacent micro-ops don't directly compete.
 //
 // MicroOpBufferSize > 1 indicates that RAW dependencies can be

lib/Target/X86/X86ScheduleBtVer2.td

 }
 def : InstRW<[JWriteJVZEROALL], (instrs VZEROALL)>;
 
 def JWriteJVZEROUPPER: SchedWriteRes<[]> {
   let Latency = 46;
   let NumMicroOps = 37;
 }
 def : InstRW<[JWriteJVZEROUPPER], (instrs VZEROUPPER)>;
+
 } // SchedModel
 
+///////////////////////////////////////////////////////////////////////////////
+//  SchedWriteVariant definitions.
+///////////////////////////////////////////////////////////////////////////////
+
+let SchedModel = BtVer2Model in {
+
+def JWriteZeroLatency : SchedWriteRes<[]> {
+  let Latency = 0;
+}
+
+// Vector XOR instructions that use the same register for both source 
+// operands do not have a real dependency on the previous contents of the
+// register, and thus, do not have to wait before completing. They can be
+// optimized out at register renaming stage.
+// Reference: Section 10.8 of the "Software Optimization Guide for AMD Family
+// 15h Processors".
+// Reference: Agner's Fog "The microarchitecture of Intel, AMD and VIA CPUs",
+// Section 21.8 [Dependency-breaking instructions].
+
+def JWriteFZeroIdiom : SchedWriteVariant<[
+    SchedVar<MCSchedPredicate<ZeroIdiomPredicate>, [JWriteZeroLatency]>,
+    SchedVar<MCSchedPredicate<TruePred>,           [WriteFLogic]>
+]>;
+
+def : InstRW<[JWriteFZeroIdiom], (instrs XORPSrr, VXORPSrr, XORPDrr, VXORPDrr)>;
+
+def JWriteVZeroIdiom : SchedWriteVariant<[
+    SchedVar<MCSchedPredicate<ZeroIdiomPredicate>, [JWriteZeroLatency]>,
+    SchedVar<MCSchedPredicate<TruePred>,           [WriteVecLogicX]>
+]>;
+
+def : InstRW<[JWriteVZeroIdiom], (instrs PXORrr, VPXORrr)>;
+
+} // SchedModel

test/CodeGen/X86/sse-schedule.ll

 ; SKX-LABEL: test_xorps:
 ; SKX:       # %bb.0:
 ; SKX-NEXT:    vxorps %xmm1, %xmm0, %xmm0 # sched: [1:0.33]
 ; SKX-NEXT:    vxorps (%rdi), %xmm0, %xmm0 # sched: [7:0.50]
 ; SKX-NEXT:    retq # sched: [7:1.00]
 ;
 ; BTVER2-SSE-LABEL: test_xorps:
 ; BTVER2-SSE:       # %bb.0:
 ; BTVER2-SSE-NEXT:    xorps %xmm1, %xmm0 # sched: [1:0.50]
 ; BTVER2-SSE-NEXT:    xorps (%rdi), %xmm0 # sched: [6:1.00]
 ; BTVER2-SSE-NEXT:    retq # sched: [4:1.00]
 ;
 ; BTVER2-LABEL: test_xorps:
 ; BTVER2:       # %bb.0:
 ; BTVER2-NEXT:    vxorps %xmm1, %xmm0, %xmm0 # sched: [1:0.50]
 ; BTVER2-NEXT:    vxorps (%rdi), %xmm0, %xmm0 # sched: [6:1.00]
 ; BTVER2-NEXT:    retq # sched: [4:1.00]

 ; SKX-NEXT:    #APP
 ; SKX-NEXT:    nop # sched: [1:0.25]
 ; SKX-NEXT:    #NO_APP
 ; SKX-NEXT:    vxorps %xmm0, %xmm0, %xmm0 # sched: [1:0.33]
 ; SKX-NEXT:    retq # sched: [7:1.00]
 ;
 ; BTVER2-SSE-LABEL: test_fnop:
 ; BTVER2-SSE:       # %bb.0:
-; BTVER2-SSE-NEXT:    xorps %xmm0, %xmm0 # sched: [1:0.50]
+; BTVER2-SSE-NEXT:    xorps %xmm0, %xmm0 # sched: [0:?]
 ; BTVER2-SSE-NEXT:    #APP
 ; BTVER2-SSE-NEXT:    nop # sched: [1:0.50]
 ; BTVER2-SSE-NEXT:    #NO_APP
 ; BTVER2-SSE-NEXT:    retq # sched: [4:1.00]
 ;
 ; BTVER2-LABEL: test_fnop:
 ; BTVER2:       # %bb.0:
-; BTVER2-NEXT:    vxorps %xmm0, %xmm0, %xmm0 # sched: [1:0.50]
+; BTVER2-NEXT:    vxorps %xmm0, %xmm0, %xmm0 # sched: [0:?]
 ; BTVER2-NEXT:    #APP
 ; BTVER2-NEXT:    nop # sched: [1:0.50]
 ; BTVER2-NEXT:    #NO_APP
 ; BTVER2-NEXT:    retq # sched: [4:1.00]
 ;
 ; ZNVER1-SSE-LABEL: test_fnop:
 ; ZNVER1-SSE:       # %bb.0:
 ; ZNVER1-SSE-NEXT:    xorps %xmm0, %xmm0 # sched: [1:0.25]

test/tools/llvm-mca/X86/BtVer2/zero-idioms.s

+# NOTE: Assertions have been autogenerated by utils/update_mca_test_checks.py
+# RUN: llvm-mca -mtriple=x86_64-unknown-unknown -mcpu=btver2 -timeline -register-file-stats -iterations=5 < %s | FileCheck %s
+
+xorps  %xmm0, %xmm0
+xorpd  %xmm1, %xmm1
+vxorps %xmm2, %xmm2, %xmm2
+vxorpd %xmm1, %xmm1, %xmm1
+pxor   %xmm2, %xmm2
+vpxor  %xmm3, %xmm3, %xmm3
+
+vxorps %xmm4, %xmm4, %xmm5
+vxorpd %xmm1, %xmm1, %xmm3
+vpxor  %xmm3, %xmm3, %xmm5
+
+# CHECK:      Iterations:        5
+# CHECK-NEXT: Instructions:      45
+# CHECK-NEXT: Total Cycles:      24
+# CHECK-NEXT: Dispatch Width:    2
+# CHECK-NEXT: IPC:               1.88
+# CHECK-NEXT: Block RThroughput: 4.5
+
+# CHECK:      Instruction Info:
+# CHECK-NEXT: [1]: #uOps
+# CHECK-NEXT: [2]: Latency
+# CHECK-NEXT: [3]: RThroughput
+# CHECK-NEXT: [4]: MayLoad
+# CHECK-NEXT: [5]: MayStore
+# CHECK-NEXT: [6]: HasSideEffects
+
+# CHECK:      [1]    [2]    [3]    [4]    [5]    [6]    Instructions:
+# CHECK-NEXT:  1      0      -                          xorps %xmm0, %xmm0
+# CHECK-NEXT:  1      0      -                          xorpd %xmm1, %xmm1
+# CHECK-NEXT:  1      0      -                          vxorps  %xmm2, %xmm2, %xmm2
+# CHECK-NEXT:  1      0      -                          vxorpd  %xmm1, %xmm1, %xmm1
+# CHECK-NEXT:  1      0      -                          pxor  %xmm2, %xmm2
+# CHECK-NEXT:  1      0      -                          vpxor %xmm3, %xmm3, %xmm3
+# CHECK-NEXT:  1      0      -                          vxorps  %xmm4, %xmm4, %xmm5
+# CHECK-NEXT:  1      0      -                          vxorpd  %xmm1, %xmm1, %xmm3
+# CHECK-NEXT:  1      0      -                          vpxor %xmm3, %xmm3, %xmm5
+
+# CHECK:      Register File statistics:
+# CHECK-NEXT: Total number of mappings created:    0
+# CHECK-NEXT: Max number of mappings used:         0
+
+# CHECK:      *  Register File #1 -- JFpuPRF:
+# CHECK-NEXT:    Number of physical registers:     72
+# CHECK-NEXT:    Total number of mappings created: 0
+# CHECK-NEXT:    Max number of mappings used:      0
+
+# CHECK:      *  Register File #2 -- JIntegerPRF:
+# CHECK-NEXT:    Number of physical registers:     64
+# CHECK-NEXT:    Total number of mappings created: 0
+# CHECK-NEXT:    Max number of mappings used:      0
+
+# CHECK:      Resources:
+# CHECK-NEXT: [0]   - JALU0
+# CHECK-NEXT: [1]   - JALU1
+# CHECK-NEXT: [2]   - JDiv
+# CHECK-NEXT: [3]   - JFPA
+# CHECK-NEXT: [4]   - JFPM
+# CHECK-NEXT: [5]   - JFPU0
+# CHECK-NEXT: [6]   - JFPU1
+# CHECK-NEXT: [7]   - JLAGU
+# CHECK-NEXT: [8]   - JMul
+# CHECK-NEXT: [9]   - JSAGU
+# CHECK-NEXT: [10]  - JSTC
+# CHECK-NEXT: [11]  - JVALU0
+# CHECK-NEXT: [12]  - JVALU1
+# CHECK-NEXT: [13]  - JVIMUL
+
+# CHECK:      Resource pressure per iteration:
+# CHECK-NEXT: [0]    [1]    [2]    [3]    [4]    [5]    [6]    [7]    [8]    [9]    [10]   [11]   [12]   [13]
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -
+
+# CHECK:      Resource pressure by instruction:
+# CHECK-NEXT: [0]    [1]    [2]    [3]    [4]    [5]    [6]    [7]    [8]    [9]    [10]   [11]   [12]   [13]   Instructions:
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     xorps %xmm0, %xmm0
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     xorpd %xmm1, %xmm1
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     vxorps  %xmm2, %xmm2, %xmm2
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     vxorpd  %xmm1, %xmm1, %xmm1
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     pxor  %xmm2, %xmm2
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     vpxor %xmm3, %xmm3, %xmm3
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     vxorps  %xmm4, %xmm4, %xmm5
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     vxorpd  %xmm1, %xmm1, %xmm3
+# CHECK-NEXT:  -      -      -      -      -      -      -      -      -      -      -      -      -      -     vpxor %xmm3, %xmm3, %xmm5
+
+# CHECK:      Timeline view:
+# CHECK-NEXT:                     0123456789
+# CHECK-NEXT: Index     0123456789          0123
+
+# CHECK:      [0,0]     DR   .    .    .    .  .   xorps  %xmm0, %xmm0
+# CHECK-NEXT: [0,1]     DR   .    .    .    .  .   xorpd  %xmm1, %xmm1
+# CHECK-NEXT: [0,2]     .DR  .    .    .    .  .   vxorps %xmm2, %xmm2, %xmm2
+# CHECK-NEXT: [0,3]     .DR  .    .    .    .  .   vxorpd %xmm1, %xmm1, %xmm1
+# CHECK-NEXT: [0,4]     . DR .    .    .    .  .   pxor %xmm2, %xmm2
+# CHECK-NEXT: [0,5]     . DR .    .    .    .  .   vpxor  %xmm3, %xmm3, %xmm3
+# CHECK-NEXT: [0,6]     .  DR.    .    .    .  .   vxorps %xmm4, %xmm4, %xmm5
+# CHECK-NEXT: [0,7]     .  DR.    .    .    .  .   vxorpd %xmm1, %xmm1, %xmm3
+# CHECK-NEXT: [0,8]     .   DR    .    .    .  .   vpxor  %xmm3, %xmm3, %xmm5
+# CHECK-NEXT: [1,0]     .   DR    .    .    .  .   xorps  %xmm0, %xmm0
+# CHECK-NEXT: [1,1]     .    DR   .    .    .  .   xorpd  %xmm1, %xmm1
+# CHECK-NEXT: [1,2]     .    DR   .    .    .  .   vxorps %xmm2, %xmm2, %xmm2
+# CHECK-NEXT: [1,3]     .    .DR  .    .    .  .   vxorpd %xmm1, %xmm1, %xmm1
+# CHECK-NEXT: [1,4]     .    .DR  .    .    .  .   pxor %xmm2, %xmm2
+# CHECK-NEXT: [1,5]     .    . DR .    .    .  .   vpxor  %xmm3, %xmm3, %xmm3
+# CHECK-NEXT: [1,6]     .    . DR .    .    .  .   vxorps %xmm4, %xmm4, %xmm5
+# CHECK-NEXT: [1,7]     .    .  DR.    .    .  .   vxorpd %xmm1, %xmm1, %xmm3
+# CHECK-NEXT: [1,8]     .    .  DR.    .    .  .   vpxor  %xmm3, %xmm3, %xmm5
+# CHECK-NEXT: [2,0]     .    .   DR    .    .  .   xorps  %xmm0, %xmm0
+# CHECK-NEXT: [2,1]     .    .   DR    .    .  .   xorpd  %xmm1, %xmm1
+# CHECK-NEXT: [2,2]     .    .    DR   .    .  .   vxorps %xmm2, %xmm2, %xmm2
+# CHECK-NEXT: [2,3]     .    .    DR   .    .  .   vxorpd %xmm1, %xmm1, %xmm1
+# CHECK-NEXT: [2,4]     .    .    .DR  .    .  .   pxor %xmm2, %xmm2
+# CHECK-NEXT: [2,5]     .    .    .DR  .    .  .   vpxor  %xmm3, %xmm3, %xmm3
+# CHECK-NEXT: [2,6]     .    .    . DR .    .  .   vxorps %xmm4, %xmm4, %xmm5
+# CHECK-NEXT: [2,7]     .    .    . DR .    .  .   vxorpd %xmm1, %xmm1, %xmm3
+# CHECK-NEXT: [2,8]     .    .    .  DR.    .  .   vpxor  %xmm3, %xmm3, %xmm5
+# CHECK-NEXT: [3,0]     .    .    .  DR.    .  .   xorps  %xmm0, %xmm0
+# CHECK-NEXT: [3,1]     .    .    .   DR    .  .   xorpd  %xmm1, %xmm1
+# CHECK-NEXT: [3,2]     .    .    .   DR    .  .   vxorps %xmm2, %xmm2, %xmm2
+# CHECK-NEXT: [3,3]     .    .    .    DR   .  .   vxorpd %xmm1, %xmm1, %xmm1
+# CHECK-NEXT: [3,4]     .    .    .    DR   .  .   pxor %xmm2, %xmm2
+# CHECK-NEXT: [3,5]     .    .    .    .DR  .  .   vpxor  %xmm3, %xmm3, %xmm3
+# CHECK-NEXT: [3,6]     .    .    .    .DR  .  .   vxorps %xmm4, %xmm4, %xmm5
+# CHECK-NEXT: [3,7]     .    .    .    . DR .  .   vxorpd %xmm1, %xmm1, %xmm3
+# CHECK-NEXT: [3,8]     .    .    .    . DR .  .   vpxor  %xmm3, %xmm3, %xmm5
+# CHECK-NEXT: [4,0]     .    .    .    .  DR.  .   xorps  %xmm0, %xmm0
+# CHECK-NEXT: [4,1]     .    .    .    .  DR.  .   xorpd  %xmm1, %xmm1
+# CHECK-NEXT: [4,2]     .    .    .    .   DR  .   vxorps %xmm2, %xmm2, %xmm2
+# CHECK-NEXT: [4,3]     .    .    .    .   DR  .   vxorpd %xmm1, %xmm1, %xmm1
+# CHECK-NEXT: [4,4]     .    .    .    .    DR .   pxor %xmm2, %xmm2
+# CHECK-NEXT: [4,5]     .    .    .    .    DR .   vpxor  %xmm3, %xmm3, %xmm3
+# CHECK-NEXT: [4,6]     .    .    .    .    .DR.   vxorps %xmm4, %xmm4, %xmm5
+# CHECK-NEXT: [4,7]     .    .    .    .    .DR.   vxorpd %xmm1, %xmm1, %xmm3
+# CHECK-NEXT: [4,8]     .    .    .    .    . DR   vpxor  %xmm3, %xmm3, %xmm5
+
+# CHECK:      Average Wait times (based on the timeline view):
+# CHECK-NEXT: [0]: Executions
+# CHECK-NEXT: [1]: Average time spent waiting in a scheduler's queue
+# CHECK-NEXT: [2]: Average time spent waiting in a scheduler's queue while ready
+# CHECK-NEXT: [3]: Average time elapsed from WB until retire stage
+
+# CHECK:            [0]    [1]    [2]    [3]
+# CHECK-NEXT: 0.     5     0.0    0.0    0.0       xorps  %xmm0, %xmm0
+# CHECK-NEXT: 1.     5     0.0    0.0    0.0       xorpd  %xmm1, %xmm1
+# CHECK-NEXT: 2.     5     0.0    0.0    0.0       vxorps %xmm2, %xmm2, %xmm2
+# CHECK-NEXT: 3.     5     0.0    0.0    0.0       vxorpd %xmm1, %xmm1, %xmm1
+# CHECK-NEXT: 4.     5     0.0    0.0    0.0       pxor %xmm2, %xmm2
+# CHECK-NEXT: 5.     5     0.0    0.0    0.0       vpxor  %xmm3, %xmm3, %xmm3
+# CHECK-NEXT: 6.     5     0.0    0.0    0.0       vxorps %xmm4, %xmm4, %xmm5
+# CHECK-NEXT: 7.     5     0.0    0.0    0.0       vxorpd %xmm1, %xmm1, %xmm3
+# CHECK-NEXT: 8.     5     0.0    0.0    0.0       vpxor  %xmm3, %xmm3, %xmm5
+

tools/llvm-mca/InstrBuilder.h

 /// Information from the machine scheduling model is used to identify processor
 /// resources that are consumed by an instruction.
 class InstrBuilder {
   const llvm::MCSubtargetInfo &STI;
   const llvm::MCInstrInfo &MCII;
   llvm::SmallVector<uint64_t, 8> ProcResourceMasks;
 
   llvm::DenseMap<unsigned short, std::unique_ptr<const InstrDesc>> Descriptors;
+  llvm::DenseMap<const llvm::MCInst *, std::unique_ptr<const InstrDesc>>
+      VariantDescriptors;
 
   const InstrDesc &createInstrDescImpl(const llvm::MCInst &MCI);
-
   InstrBuilder(const InstrBuilder &) = delete;
   InstrBuilder &operator=(const InstrBuilder &) = delete;
 
 public:
   InstrBuilder(const llvm::MCSubtargetInfo &sti, const llvm::MCInstrInfo &mcii)
       : STI(sti), MCII(mcii),
         ProcResourceMasks(STI.getSchedModel().getNumProcResourceKinds()) {
     computeProcResourceMasks(STI.getSchedModel(), ProcResourceMasks);

tools/llvm-mca/InstrBuilder.cpp

const InstrDesc &InstrBuilder::createInstrDescImpl(const MCInst &MCI) {
 
   unsigned short Opcode = MCI.getOpcode();
   // Obtain the instruction descriptor from the opcode.
   const MCInstrDesc &MCDesc = MCII.get(Opcode);
   const MCSchedModel &SM = STI.getSchedModel();
 
   // Then obtain the scheduling class information from the instruction.
   unsigned SchedClassID = MCDesc.getSchedClass();
-  const MCSchedClassDesc &SCDesc = *SM.getSchedClassDesc(SchedClassID);
+  unsigned CPUID = SM.getProcessorID();
+
+  // Try to solve variant scheduling classes.
+  while (SchedClassID && SM.getSchedClassDesc(SchedClassID)->isVariant())
+    SchedClassID = STI.resolveVariantSchedClass(SchedClassID, &MCI, CPUID);
+
+  if (!SchedClassID)
+    llvm::report_fatal_error("unable to resolve this variant class.");
 
   // Create a new empty descriptor.
   std::unique_ptr<InstrDesc> ID = llvm::make_unique<InstrDesc>();
 
-  if (SCDesc.isVariant()) {
-    WithColor::warning() << "don't know how to model variant opcodes.\n";
-    WithColor::note() << "assume 1 micro opcode.\n";
-    ID->NumMicroOps = 1U;
-  } else {
-    ID->NumMicroOps = SCDesc.NumMicroOps;
-  }
+  const MCSchedClassDesc &SCDesc = *SM.getSchedClassDesc(SchedClassID);
 
   if (MCDesc.isCall()) {
     // We don't correctly model calls.
     WithColor::warning() << "found a call in the input assembly sequence.\n";
     WithColor::note() << "call instructions are not correctly modeled. "
                       << "Assume a latency of 100cy.\n";
   }
 
const InstrDesc &InstrBuilder::createInstrDescImpl(const MCInst &MCI) {
   computeMaxLatency(*ID, MCDesc, SCDesc, STI);
   populateWrites(*ID, MCI, MCDesc, SCDesc, STI);
   populateReads(*ID, MCI, MCDesc, SCDesc, STI);
 
   LLVM_DEBUG(dbgs() << "\t\tMaxLatency=" << ID->MaxLatency << '\n');
   LLVM_DEBUG(dbgs() << "\t\tNumMicroOps=" << ID->NumMicroOps << '\n');
 
   // Now add the new descriptor.
-  Descriptors[Opcode] = std::move(ID);
-  return *Descriptors[Opcode];
+  SchedClassID = MCDesc.getSchedClass();
+  if (!SM.getSchedClassDesc(SchedClassID)->isVariant()) {
+    Descriptors[MCI.getOpcode()] = std::move(ID);
+    return *Descriptors[MCI.getOpcode()];
+  }
+
+  VariantDescriptors[&MCI] = std::move(ID);
+  return *VariantDescriptors[&MCI];
 }
 
 const InstrDesc &InstrBuilder::getOrCreateInstrDesc(const MCInst &MCI) {
-  if (Descriptors.find_as(MCI.getOpcode()) == Descriptors.end())
-    return createInstrDescImpl(MCI);
+  if (Descriptors.find_as(MCI.getOpcode()) != Descriptors.end())
     return *Descriptors[MCI.getOpcode()];
+
+  if (VariantDescriptors.find(&MCI) != VariantDescriptors.end())
+    return *VariantDescriptors[&MCI];
+
+  return createInstrDescImpl(MCI);
 }
 
 std::unique_ptr<Instruction>
 InstrBuilder::createInstruction(const MCInst &MCI) {
   const InstrDesc &D = getOrCreateInstrDesc(MCI);
   std::unique_ptr<Instruction> NewIS = llvm::make_unique<Instruction>(D);
 
   // Initialize Reads first.

tools/llvm-mca/InstructionInfoView.cpp

void InstructionInfoView::printView(raw_ostream &OS) const {
   TempStream << "\n\nInstruction Info:\n";
   TempStream << "[1]: #uOps\n[2]: Latency\n[3]: RThroughput\n"
              << "[4]: MayLoad\n[5]: MayStore\n[6]: HasSideEffects\n\n";
 
   TempStream << "[1]    [2]    [3]    [4]    [5]    [6]    Instructions:\n";
   for (unsigned I = 0, E = Instructions; I < E; ++I) {
     const MCInst &Inst = Source.getMCInstFromIndex(I);
     const MCInstrDesc &MCDesc = MCII.get(Inst.getOpcode());
-    const MCSchedClassDesc &SCDesc =
-        *SM.getSchedClassDesc(MCDesc.getSchedClass());
+
+    // Obtain the scheduling class information from the instruction.
+    unsigned SchedClassID = MCDesc.getSchedClass();
+    unsigned CPUID = SM.getProcessorID();
+
+    // Try to solve variant scheduling classes.
+    while (SchedClassID && SM.getSchedClassDesc(SchedClassID)->isVariant())
+      SchedClassID = STI.resolveVariantSchedClass(SchedClassID, &Inst, CPUID);
+
+    assert(SchedClassID && "Unable to solve this variant scheduling class!");
+    const MCSchedClassDesc &SCDesc = *SM.getSchedClassDesc(SchedClassID);
 
     unsigned NumMicroOpcodes = SCDesc.NumMicroOps;
     unsigned Latency = MCSchedModel::computeInstrLatency(STI, SCDesc);
     Optional<double> RThroughput =
         MCSchedModel::getReciprocalThroughput(STI, SCDesc);
 
     TempStream << ' ' << NumMicroOpcodes << "    ";
     if (NumMicroOpcodes < 10)