llvm-reduce: Add pass to reduce MIR instruction flags

[MatzeB]

Removing instruction flags seems dangerous to me:

• Dropping BundledPred, BundledSucc will unbundle instructions and leave a bundle header around in the instruction stream, which seems bad.
• I wouldn't be surprised if some targets break/segfault when FrameSetup/FrameDestroy is dropped randomly.

I could see this work okay for the math and overflow flags. Maybe it would be best to explicitely list those "mostly harmless" flags here and leave the others untouched?

llvm/tools/llvm-reduce/ReducerWorkItem.cpp

247

Nice catch.

[MatzeB]

llvm/tools/llvm-reduce/ReducerWorkItem.cpp

247

Feel free to just push this independently without separate review.

[arsenm]

In D124044#3476361, @MatzeB wrote:

Removing instruction flags seems dangerous to me:

• Dropping BundledPred, BundledSucc will unbundle instructions and leave a bundle header around in the instruction stream, which seems bad.
• I wouldn't be surprised if some targets break/segfault when FrameSetup/FrameDestroy is dropped randomly.

I forgot these were stored as part of flags. However, I think I accidentally avoided this bug. setFlags actually does take care to preserve the BundledPred/BundledSucc:

void setFlags(unsigned flags) {
   // Filter out the automatically maintained flags.
   unsigned Mask = BundledPred | BundledSucc;
   Flags = (Flags & Mask) | (flags & ~Mask);
 }

I guess there still is potentially a bug from assuming getFlags != 0 can be reduced further, although it should at worst cause an avoidable reduction attempt.
As far as FrameSetup/FrameDestroy are concerned, it's a bug if the targets break on them. Unless there's a verifier enforced property, targets should be able to handle these unset

llvm/tools/llvm-reduce/ReducerWorkItem.cpp

247

Also forgot about AsmPrinterFlags

[arsenm]

ping

[MatzeB]

As far as FrameSetup/FrameDestroy are concerned, it's a bug if the targets break on them.

You can probably debate that with target authors :)... Anyway I don't have strong feelings and this seems valuable to move forward with.

LGTM

[arsenm]

In D124044#3523534, @MatzeB wrote:

As far as FrameSetup/FrameDestroy are concerned, it's a bug if the targets break on them.

You can probably debate that with target authors :)... Anyway I don't have strong feelings and this seems valuable to move forward with.

If there's some required property, it would need to be checked by the verifier. If it's not checked by the verifier, you can't rely on it. Cases that fail the verifier are filtered out

[arsenm]

2011052150e1a2fb8586b030126f8bec338f4cc5

llvm/test/tools/llvm-reduce/mir/reduce-instruction-flags.mir

+# REQUIRES: amdgpu-registered-target
+# RUN: llvm-reduce -simplify-mir -mtriple=amdgcn-amd-amdhsa --test FileCheck --test-arg --check-prefix=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t 2> %t.log
+# RUN: FileCheck --check-prefix=RESULT %s < %t
+
+# CHECK-INTERESTINGNESS: V_ADD_F32
+# CHECK-INTERESTINGNESS: nnan nofpexcept V_MUL_F32
+
+# CHECK-INTERESTINGNESS-COUNT-11: V_MOV_B32
+
+
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_ADD_F32_e32 %{{[0-9]+}}, %{{[0-9]+}}, implicit $mode, implicit $exec
+# RESULT: %{{[0-9]+}}:vgpr_32 = nnan nofpexcept V_MUL_F32_e32 0, %{{[0-9]+}}, implicit $mode, implicit $exec
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+# RESULT: %{{[0-9]+}}:vgpr_32 = V_MOV_B32_e32
+
+---
+name: func
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $vgpr1
+
+    S_WAITCNT 0
+    %0:vgpr_32 = COPY $vgpr0
+    %1:vgpr_32 = COPY $vgpr1
+    %2:vgpr_32 = nofpexcept V_ADD_F32_e32 %0, %1, implicit $mode, implicit $exec
+    %3:vgpr_32 = nnan nofpexcept V_MUL_F32_e32 0, %2, implicit $mode, implicit $exec
+    %4:vgpr_32 = nsz V_MUL_F32_e32 0, %3, implicit $mode, implicit $exec
+
+    %5:vgpr_32 = nnan V_MOV_B32_e32 0, implicit $exec
+    %6:vgpr_32 = ninf V_MOV_B32_e32 0, implicit $exec
+    %7:vgpr_32 = nsz V_MOV_B32_e32 0, implicit $exec
+    %8:vgpr_32 = arcp V_MOV_B32_e32 0, implicit $exec
+    %9:vgpr_32 = contract V_MOV_B32_e32 0, implicit $exec
+    %10:vgpr_32 = afn V_MOV_B32_e32 0, implicit $exec
+    %11:vgpr_32 = reassoc V_MOV_B32_e32 0, implicit $exec
+    %12:vgpr_32 = nuw V_MOV_B32_e32 0, implicit $exec
+    %13:vgpr_32 = nsw V_MOV_B32_e32 0, implicit $exec
+    %14:vgpr_32 = exact V_MOV_B32_e32 0, implicit $exec
+    %15:vgpr_32 = nofpexcept V_MOV_B32_e32 0, implicit $exec
+    S_NOP 0, implicit %3, implicit %4, implicit %5, implicit %6, implicit %7
+    S_NOP 0, implicit %8, implicit %9, implicit %10, implicit %11, implicit %12
+    S_ENDPGM 0, implicit %13, implicit %14, implicit %15
+...
+

llvm/tools/llvm-reduce/CMakeLists.txt

add_llvm_tool(llvm-reduce
   deltas/ReduceMetadata.cpp
   deltas/ReduceModuleData.cpp
   deltas/ReduceOperandBundles.cpp
   deltas/ReduceSpecialGlobals.cpp
   deltas/ReduceOperands.cpp
   deltas/ReduceOperandsSkip.cpp
   deltas/ReduceOperandsToArgs.cpp
   deltas/ReduceInstructionsMIR.cpp
+  deltas/ReduceInstructionFlagsMIR.cpp
   deltas/ReduceIRReferences.cpp
   llvm-reduce.cpp
 
   DEPENDS
   intrinsics_gen
   )

llvm/tools/llvm-reduce/DeltaManager.cpp

 #include "deltas/ReduceBasicBlocks.h"
 #include "deltas/ReduceFunctionBodies.h"
 #include "deltas/ReduceFunctions.h"
 #include "deltas/ReduceGlobalObjects.h"
 #include "deltas/ReduceGlobalValues.h"
 #include "deltas/ReduceGlobalVarInitializers.h"
 #include "deltas/ReduceGlobalVars.h"
 #include "deltas/ReduceIRReferences.h"
+#include "deltas/ReduceInstructionFlagsMIR.h"
 #include "deltas/ReduceInstructions.h"
 #include "deltas/ReduceInstructionsMIR.h"
 #include "deltas/ReduceMetadata.h"
 #include "deltas/ReduceModuleData.h"
 #include "deltas/ReduceOperandBundles.h"
 #include "deltas/ReduceOperands.h"
 #include "deltas/ReduceOperandsSkip.h"
 #include "deltas/ReduceOperandsToArgs.h"
#define DELTA_PASSES                                                           \
   DELTA_PASS("attributes", reduceAttributesDeltaPass)                          \
   DELTA_PASS("module-data", reduceModuleDataDeltaPass)
 
 #define DELTA_PASSES_MIR                                                       \
   DELTA_PASS("instructions", reduceInstructionsMIRDeltaPass)                   \
   DELTA_PASS("ir-instruction-references",                                      \
              reduceIRInstructionReferencesDeltaPass)                           \
   DELTA_PASS("ir-block-references", reduceIRBlockReferencesDeltaPass)          \
-  DELTA_PASS("ir-function-references", reduceIRFunctionReferencesDeltaPass)
+  DELTA_PASS("ir-function-references", reduceIRFunctionReferencesDeltaPass)    \
+  DELTA_PASS("instruction-flags", reduceInstructionFlagsMIRDeltaPass)
 
 static void runAllDeltaPasses(TestRunner &Tester) {
 #define DELTA_PASS(NAME, FUNC) FUNC(Tester);
   if (Tester.getProgram().isMIR()) {
     DELTA_PASSES_MIR
   } else {
     DELTA_PASSES
   }

llvm/tools/llvm-reduce/ReducerWorkItem.cpp

static std::unique_ptr<MachineFunction> cloneMF(MachineFunction *SrcMF) {
   // Clone instructions.
   for (auto &SrcMBB : *SrcMF) {
     auto *DstMBB = Src2DstMBB[&SrcMBB];
     for (auto &SrcMI : SrcMBB) {
       const auto &MCID =
           DstMF->getSubtarget().getInstrInfo()->get(SrcMI.getOpcode());
       auto *DstMI = DstMF->CreateMachineInstr(MCID, SrcMI.getDebugLoc(),
                                               /*NoImplicit=*/true);
+      DstMI->setFlags(SrcMI.getFlags());
+
       DstMBB->push_back(DstMI);
       for (auto &SrcMO : SrcMI.operands()) {
         MachineOperand DstMO(SrcMO);
         DstMO.clearParent();
 
         // Update MBB.
         if (DstMO.isMBB())
           DstMO.setMBB(Src2DstMBB[DstMO.getMBB()]);

llvm/tools/llvm-reduce/deltas/ReduceInstructionFlagsMIR.h

+//===- ReduceInstructionFlagsMIR.h  - Specialized Delta Pass --------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a function which calls the Generic Delta pass in order
+// to reduce uninteresting MachineInstr flags from the MachineFunction.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TOOLS_LLVM_REDUCE_DELTAS_REDUCEINSTRUCTIONFLAGSMIR_H
+#define LLVM_TOOLS_LLVM_REDUCE_DELTAS_REDUCEINSTRUCTIONFLAGSMIR_H
+
+#include "Delta.h"
+
+namespace llvm {
+void reduceInstructionFlagsMIRDeltaPass(TestRunner &Test);
+} // namespace llvm
+
+#endif

llvm/tools/llvm-reduce/deltas/ReduceInstructionFlagsMIR.cpp

+//===- ReduceInstructionFlagsMIR.cpp - Specialized Delta Pass -------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a function which calls the Generic Delta pass in order
+// to reduce uninteresting MachineInstr flags from the MachineFunction.
+//
+//===----------------------------------------------------------------------===//
+
+#include "ReduceInstructionFlagsMIR.h"
+#include "llvm/CodeGen/MachineFunction.h"
+using namespace llvm;
+
+static void removeFlagsFromModule(Oracle &O, ReducerWorkItem &WorkItem) {
+  for (const Function &F : WorkItem.getModule()) {
+    if (auto *MF = WorkItem.MMI->getMachineFunction(F)) {
+      for (MachineBasicBlock &MBB : *MF) {
+        for (MachineInstr &MI : MBB) {
+          // TODO: Should this clear flags individually?
+          if (MI.getFlags() != 0 && !O.shouldKeep())
+            MI.setFlags(0);
+        }
+      }
+    }
+  }
+}
+
+void llvm::reduceInstructionFlagsMIRDeltaPass(TestRunner &Test) {
+  outs() << "*** Reducing Instruction flags...\n";
+  runDeltaPass(Test, removeFlagsFromModule);
+}