[MachineSink] Add a loop sink limit

[SjoerdMeijer]

Summary

To make sure compile-times don't regress, add an option to restrict the number of instructions considered for sinking as alias analysis can be expensive and for the same reason also skip large blocks.

[shchenz]

llvm/test/CodeGen/AArch64/loop-sink.mir

1052 ↗

(On Diff #322940)

Is this expected? Not sure why this is impacted by the patch. Seems the sinking number are just the same as we have same instruction sequence except for the killed flag

[SjoerdMeijer]

Thanks for checking. The unrelated change is gone now.
Locally I had this patch applied on top of D94308, which explains that.

[shchenz]

In D96485#2565535, @SjoerdMeijer wrote:

Thanks for checking. The unrelated change is gone now.
Locally I had this patch applied on top of D94308, which explains that.

Thanks! LGTM.

llvm/lib/CodeGen/MachineSink.cpp

static cl::opt<unsigned> SinkLoadBlocksThreshold(
     cl::init(20), cl::Hidden);
 
 static cl::opt<bool>
 SinkInstsIntoLoop("sink-insts-to-avoid-spills",
                   cl::desc("Sink instructions into loops to avoid "
                            "register spills"),
                   cl::init(false), cl::Hidden);
 
+static cl::opt<unsigned> SinkIntoLoopLimit(
+    "machine-sink-loop-limit",
+    cl::desc("The maximum number of instructions considered for loop sinking."),
+    cl::init(50), cl::Hidden);
+
 STATISTIC(NumSunk,      "Number of machine instructions sunk");
 STATISTIC(NumLoopSunk,  "Number of machine instructions sunk into a loop");
 STATISTIC(NumSplit,     "Number of critical edges split");
 STATISTIC(NumCoalesces, "Number of copies coalesced");
 STATISTIC(NumPostRACopySink, "Number of copies sunk after RA");
 
 namespace {
 
for (auto *L : Loops) {
         LLVM_DEBUG(dbgs() << "LoopSink: Can't find preheader\n");
         continue;
       }
       SmallVector<MachineInstr *, 8> Candidates;
       FindLoopSinkCandidates(L, Preheader, Candidates);
 
       // Walk the candidates in reverse order so that we start with the use
       // of a def-use chain, if there is any.
+      // TODO: Sort the candidates using a cost-model.
+      unsigned i = 0;
       for (auto It = Candidates.rbegin(); It != Candidates.rend(); ++It) {
+        if (i++ == SinkIntoLoopLimit) {
+          LLVM_DEBUG(dbgs() << "LoopSink:   Limit reached of instructions to "
+                               "be analysed.");
+          break;
+        }
+
         MachineInstr *I = *It;
         if (!SinkIntoLoop(L, *I))
           break;
         EverMadeChange = true;
         ++NumLoopSunk;
       }
     }
   }
bool MachineSinking::SinkIntoLoop(MachineLoop *L, MachineInstr &I) {
   if (!SinkBlock) {
     LLVM_DEBUG(dbgs() << "LoopSink: Not sinking, can't find sink block.\n");
     return false;
   }
   if (SinkBlock == Preheader) {
     LLVM_DEBUG(dbgs() << "LoopSink: Not sinking, sink block is the preheader\n");
     return false;
   }
+  if (SinkBlock->size() > SinkLoadInstsPerBlockThreshold) {
+    LLVM_DEBUG(dbgs() << "LoopSink: Not Sinking, block too large to analyse.\n");
+    return false;
+  }
 
   LLVM_DEBUG(dbgs() << "LoopSink: Sinking instruction!\n");
   SinkBlock->splice(SinkBlock->getFirstNonPHI(), Preheader, I);
 
   // The instruction is moved from its basic block, so do not retain the
   // debug information.
   assert(!I.isDebugInstr() && "Should not sink debug inst");
   I.setDebugLoc(DebugLoc());

llvm/test/CodeGen/AArch64/loop-sink-limit.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64 -run-pass=machine-sink -sink-insts-to-avoid-spills \
+# RUN:   -machine-sink-loop-limit=1 -verify-machineinstrs %s -o - 2>&1 | \
+# RUN:   FileCheck %s --check-prefix=SINK1
+#
+# RUN: llc -mtriple aarch64 -run-pass=machine-sink -sink-insts-to-avoid-spills \
+# RUN:   -machine-sink-loop-limit=2 -verify-machineinstrs %s -o - 2>&1 | \
+# RUN:   FileCheck %s --check-prefix=SINK2
+
+--- |
+  target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+  target triple = "aarch64"
+
+  @A = external dso_local global [100 x i32], align 4
+  %struct.A = type { i32, i32, i32, i32, i32, i32 }
+
+  define i32 @do_sink_use_is_not_a_copy(i32 %n) {
+  entry:
+    %cmp63 = icmp sgt i32 %n, 0
+    br i1 %cmp63, label %for.body.preheader, label %for.cond.cleanup
+
+  for.body.preheader:                               ; preds = %entry
+    %0 = load i32, i32* getelementptr inbounds ([100 x i32], [100 x i32]* @A, i64 0, i64 0), align 4
+    br label %for.body
+
+  for.cond.cleanup:                                 ; preds = %for.body, %entry
+    %sum.0.lcssa = phi i32 [ %n, %entry ], [ %div, %for.body ]
+    ret i32 %sum.0.lcssa
+
+  for.body:                                         ; preds = %for.body, %for.body.preheader
+    %lsr.iv = phi i32 [ %n, %for.body.preheader ], [ %lsr.iv.next, %for.body ]
+    %sum.065 = phi i32 [ %div, %for.body ], [ %n, %for.body.preheader ]
+    %div = sdiv i32 %sum.065, %0
+    %lsr.iv.next = add i32 %lsr.iv, -1
+    %exitcond.not = icmp eq i32 %lsr.iv.next, 0
+    br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
+  }
+
+...
+---
+name:            do_sink_use_is_not_a_copy
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:
+  - { id: 0, class: gpr32, preferred-register: '' }
+  - { id: 1, class: gpr32all, preferred-register: '' }
+  - { id: 2, class: gpr32sp, preferred-register: '' }
+  - { id: 3, class: gpr32, preferred-register: '' }
+  - { id: 4, class: gpr32all, preferred-register: '' }
+  - { id: 5, class: gpr32all, preferred-register: '' }
+  - { id: 6, class: gpr32common, preferred-register: '' }
+  - { id: 7, class: gpr32, preferred-register: '' }
+  - { id: 8, class: gpr64common, preferred-register: '' }
+  - { id: 9, class: gpr32, preferred-register: '' }
+  - { id: 10, class: gpr32, preferred-register: '' }
+  - { id: 11, class: gpr32, preferred-register: '' }
+liveins:
+  - { reg: '$w0', virtual-reg: '%6' }
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    1
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  maxCallFrameSize: 0
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      []
+stack:           []
+callSites:       []
+debugValueSubstitutions: []
+constants:       []
+machineFunctionInfo: {}
+body:             |
+  ; SINK1-LABEL: name: do_sink_use_is_not_a_copy
+  ; SINK1: bb.0.entry:
+  ; SINK1:   successors: %bb.1(0x50000000), %bb.2(0x30000000)
+  ; SINK1:   liveins: $w0
+  ; SINK1:   [[COPY:%[0-9]+]]:gpr32common = COPY $w0
+  ; SINK1:   [[SUBSWri:%[0-9]+]]:gpr32 = SUBSWri [[COPY]], 1, 0, implicit-def $nzcv
+  ; SINK1:   Bcc 11, %bb.2, implicit $nzcv
+  ; SINK1:   B %bb.1
+  ; SINK1: bb.1.for.body.preheader:
+  ; SINK1:   successors: %bb.3(0x80000000)
+  ; SINK1:   [[ADRP:%[0-9]+]]:gpr64common = ADRP target-flags(aarch64-page) @A
+  ; SINK1:   B %bb.3
+  ; SINK1: bb.2.for.cond.cleanup:
+  ; SINK1:   [[PHI:%[0-9]+]]:gpr32all = PHI [[COPY]], %bb.0, %4, %bb.3
+  ; SINK1:   $w0 = COPY [[PHI]]
+  ; SINK1:   RET_ReallyLR implicit $w0
+  ; SINK1: bb.3.for.body:
+  ; SINK1:   successors: %bb.2(0x04000000), %bb.3(0x7c000000)
+  ; SINK1:   [[PHI1:%[0-9]+]]:gpr32sp = PHI [[COPY]], %bb.1, %5, %bb.3
+  ; SINK1:   [[PHI2:%[0-9]+]]:gpr32 = PHI [[COPY]], %bb.1, %4, %bb.3
+  ; SINK1:   [[LDRWui:%[0-9]+]]:gpr32 = LDRWui [[ADRP]], target-flags(aarch64-pageoff, aarch64-nc) @A :: (dereferenceable load 4 from `i32* getelementptr inbounds ([100 x i32], [100 x i32]* @A, i64 0, i64 0)`)
+  ; SINK1:   [[SDIVWr:%[0-9]+]]:gpr32 = SDIVWr [[PHI2]], [[LDRWui]]
+  ; SINK1:   [[COPY1:%[0-9]+]]:gpr32all = COPY [[SDIVWr]]
+  ; SINK1:   [[SUBSWri1:%[0-9]+]]:gpr32 = SUBSWri [[PHI1]], 1, 0, implicit-def $nzcv
+  ; SINK1:   [[COPY2:%[0-9]+]]:gpr32all = COPY [[SUBSWri1]]
+  ; SINK1:   Bcc 0, %bb.2, implicit $nzcv
+  ; SINK1:   B %bb.3
+  ; SINK2-LABEL: name: do_sink_use_is_not_a_copy
+  ; SINK2: bb.0.entry:
+  ; SINK2:   successors: %bb.1(0x50000000), %bb.2(0x30000000)
+  ; SINK2:   liveins: $w0
+  ; SINK2:   [[COPY:%[0-9]+]]:gpr32common = COPY $w0
+  ; SINK2:   [[SUBSWri:%[0-9]+]]:gpr32 = SUBSWri [[COPY]], 1, 0, implicit-def $nzcv
+  ; SINK2:   Bcc 11, %bb.2, implicit $nzcv
+  ; SINK2:   B %bb.1
+  ; SINK2: bb.1.for.body.preheader:
+  ; SINK2:   successors: %bb.3(0x80000000)
+  ; SINK2:   B %bb.3
+  ; SINK2: bb.2.for.cond.cleanup:
+  ; SINK2:   [[PHI:%[0-9]+]]:gpr32all = PHI [[COPY]], %bb.0, %4, %bb.3
+  ; SINK2:   $w0 = COPY [[PHI]]
+  ; SINK2:   RET_ReallyLR implicit $w0
+  ; SINK2: bb.3.for.body:
+  ; SINK2:   successors: %bb.2(0x04000000), %bb.3(0x7c000000)
+  ; SINK2:   [[PHI1:%[0-9]+]]:gpr32sp = PHI [[COPY]], %bb.1, %5, %bb.3
+  ; SINK2:   [[PHI2:%[0-9]+]]:gpr32 = PHI [[COPY]], %bb.1, %4, %bb.3
+  ; SINK2:   [[ADRP:%[0-9]+]]:gpr64common = ADRP target-flags(aarch64-page) @A
+  ; SINK2:   [[LDRWui:%[0-9]+]]:gpr32 = LDRWui [[ADRP]], target-flags(aarch64-pageoff, aarch64-nc) @A :: (dereferenceable load 4 from `i32* getelementptr inbounds ([100 x i32], [100 x i32]* @A, i64 0, i64 0)`)
+  ; SINK2:   [[SDIVWr:%[0-9]+]]:gpr32 = SDIVWr [[PHI2]], [[LDRWui]]
+  ; SINK2:   [[COPY1:%[0-9]+]]:gpr32all = COPY [[SDIVWr]]
+  ; SINK2:   [[SUBSWri1:%[0-9]+]]:gpr32 = SUBSWri [[PHI1]], 1, 0, implicit-def $nzcv
+  ; SINK2:   [[COPY2:%[0-9]+]]:gpr32all = COPY [[SUBSWri1]]
+  ; SINK2:   Bcc 0, %bb.2, implicit $nzcv
+  ; SINK2:   B %bb.3
+  bb.0.entry:
+    successors: %bb.1(0x50000000), %bb.2(0x30000000)
+    liveins: $w0
+
+    %6:gpr32common = COPY $w0
+    %7:gpr32 = SUBSWri %6, 1, 0, implicit-def $nzcv
+    Bcc 11, %bb.2, implicit $nzcv
+    B %bb.1
+
+  bb.1.for.body.preheader:
+    successors: %bb.3(0x80000000)
+
+    %8:gpr64common = ADRP target-flags(aarch64-page) @A
+    %9:gpr32 = LDRWui killed %8, target-flags(aarch64-pageoff, aarch64-nc) @A :: (dereferenceable load 4 from `i32* getelementptr inbounds ([100 x i32], [100 x i32]* @A, i64 0, i64 0)`)
+    B %bb.3
+
+  bb.2.for.cond.cleanup:
+    %1:gpr32all = PHI %6, %bb.0, %4, %bb.3
+    $w0 = COPY %1
+    RET_ReallyLR implicit $w0
+
+  bb.3.for.body:
+    successors: %bb.2(0x04000000), %bb.3(0x7c000000)
+
+    %2:gpr32sp = PHI %6, %bb.1, %5, %bb.3
+    %3:gpr32 = PHI %6, %bb.1, %4, %bb.3
+    %10:gpr32 = SDIVWr %3, %9
+    %4:gpr32all = COPY %10
+    %11:gpr32 = SUBSWri %2, 1, 0, implicit-def $nzcv
+    %5:gpr32all = COPY %11
+    Bcc 0, %bb.2, implicit $nzcv
+    B %bb.3
+
+...