[HWLoops] Stop converting to a while loop when it would be unsafe to

[samtebbs]

Summary

There are cases where a do-while loop would be converted to a while loop before finding out that it would be unsafe to expand the SCEV in this situation and then bailing out of hardware loop conversion.

This patch checks if it would be unsafe to expand the SCEV and if so stops converting the do-while into a while, allowing conversion to a hardware loop.

[samtebbs]

Add context to diff

[samparker]

Nice and tidy fix, thanks!

llvm/lib/CodeGen/HardwareLoops.cpp

406

nit: I think we're supposed to just use auto when it's obvious what the type is, say when using a cast.

411

nit: you don't need curly brackets for one liners.

[samtebbs]

Fix formatting

[samtebbs]

In D83953#2157642, @samparker wrote:

Nice and tidy fix, thanks!

Thank you!

[samtebbs]

Update tail predication option in test

llvm/lib/CodeGen/HardwareLoops.cpp

if (SE.isLoopEntryGuardedByCond(L, ICmpInst::ICMP_NE, ExitCount,
                                   SE.getZero(ExitCount->getType()))) {
     LLVM_DEBUG(dbgs() << " - Attempting to use test.set counter.\n");
     UseLoopGuard |= ForceGuardLoopEntry;
   } else
     UseLoopGuard = false;
 
   BasicBlock *BB = L->getLoopPreheader();
   if (UseLoopGuard && BB->getSinglePredecessor() &&
-      cast<BranchInst>(BB->getTerminator())->isUnconditional())
-    BB = BB->getSinglePredecessor();
+      cast<BranchInst>(BB->getTerminator())->isUnconditional()) {
+    BasicBlock *Predecessor = BB->getSinglePredecessor();
+    // If it's not safe to create a while loop then don't force it and create a
+    // do-while loop instead
+    if (!isSafeToExpandAt(ExitCount, Predecessor->getTerminator(), SE))
+        UseLoopGuard = false;
+    else
+        BB = Predecessor;
+  }
 
   if (!isSafeToExpandAt(ExitCount, BB->getTerminator(), SE)) {
     LLVM_DEBUG(dbgs() << "- Bailing, unsafe to expand ExitCount "
                << *ExitCount << "\n");
     return nullptr;
   }
 
   Value *Count = SCEVE.expandCodeFor(ExitCount, CountType,

llvm/test/CodeGen/Thumb2/LowOverheadLoops/exitcount.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=thumbv8.1m.main-none-none-eabi -mattr=+mve.fp -verify-machineinstrs -tail-predication=enabled -o - %s | FileCheck %s
+%struct.SpeexPreprocessState_ = type { i32, i32, half*, half* }
+
+define void @foo(%struct.SpeexPreprocessState_* nocapture readonly %st, i16* %x) {
+; CHECK-LABEL: foo:
+; CHECK:       @ %bb.0: @ %entry
+; CHECK:    dlstp.16 lr, r4
+; CHECK-NEXT:  .LBB0_1: @ %do.body
+; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    vldrh.u16 q0, [r2], #16
+; CHECK-NEXT:    vstrh.16 q0, [r3], #16
+; CHECK-NEXT:    letp lr, .LBB0_1
+; CHECK:    dlstp.16 lr, r3
+; CHECK-NEXT:  .LBB0_3: @ %do.body6
+; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    vldrh.u16 q1, [r1], #16
+; CHECK-NEXT:    vcvt.f16.s16 q1, q1
+; CHECK-NEXT:    vmul.f16 q1, q1, q0
+; CHECK-NEXT:    vstrh.16 q1, [r0], #16
+; CHECK-NEXT:    letp lr, .LBB0_3
+; CHECK-NEXT:  @ %bb.4: @ %do.end13
+; CHECK-NEXT:    pop {r4, pc}
+entry:
+  %ps_size = getelementptr inbounds %struct.SpeexPreprocessState_, %struct.SpeexPreprocessState_* %st, i32 0, i32 1
+  %0 = load i32, i32* %ps_size, align 4
+  %mul = shl nsw i32 %0, 1
+  %frame_size = getelementptr inbounds %struct.SpeexPreprocessState_, %struct.SpeexPreprocessState_* %st, i32 0, i32 0
+  %1 = load i32, i32* %frame_size, align 4
+  %sub = sub nsw i32 %mul, %1
+  %inbuf = getelementptr inbounds %struct.SpeexPreprocessState_, %struct.SpeexPreprocessState_* %st, i32 0, i32 3
+  %2 = load half*, half** %inbuf, align 4
+  %frame = getelementptr inbounds %struct.SpeexPreprocessState_, %struct.SpeexPreprocessState_* %st, i32 0, i32 2
+  %3 = load half*, half** %frame, align 4
+  br label %do.body
+
+do.body:                                          ; preds = %do.body, %entry
+  %pinbuff16.0 = phi half* [ %2, %entry ], [ %add.ptr, %do.body ]
+  %blkCnt.0 = phi i32 [ %sub, %entry ], [ %sub2, %do.body ]
+  %pframef16.0 = phi half* [ %3, %entry ], [ %add.ptr1, %do.body ]
+  %4 = tail call <8 x i1> @llvm.arm.mve.vctp16(i32 %blkCnt.0)
+  %5 = bitcast half* %pinbuff16.0 to <8 x half>*
+  %6 = tail call fast <8 x half> @llvm.masked.load.v8f16.p0v8f16(<8 x half>* %5, i32 2, <8 x i1> %4, <8 x half> zeroinitializer)
+  %7 = bitcast half* %pframef16.0 to <8 x half>*
+  tail call void @llvm.masked.store.v8f16.p0v8f16(<8 x half> %6, <8 x half>* %7, i32 2, <8 x i1> %4)
+  %add.ptr = getelementptr inbounds half, half* %pinbuff16.0, i32 8
+  %add.ptr1 = getelementptr inbounds half, half* %pframef16.0, i32 8
+  %sub2 = add nsw i32 %blkCnt.0, -8
+  %cmp = icmp sgt i32 %blkCnt.0, 8
+  br i1 %cmp, label %do.body, label %do.end
+
+do.end:                                           ; preds = %do.body
+  %8 = load half*, half** %frame, align 4
+  %add.ptr4 = getelementptr inbounds half, half* %8, i32 %sub
+  %9 = load i32, i32* %frame_size, align 4
+  br label %do.body6
+
+do.body6:                                         ; preds = %do.body6, %do.end
+  %px.0 = phi i16* [ %x, %do.end ], [ %add.ptr8, %do.body6 ]
+  %blkCnt.1 = phi i32 [ %9, %do.end ], [ %sub10, %do.body6 ]
+  %pframef16.1 = phi half* [ %add.ptr4, %do.end ], [ %add.ptr9, %do.body6 ]
+  %10 = tail call <8 x i1> @llvm.arm.mve.vctp16(i32 %blkCnt.1)
+  %11 = bitcast i16* %px.0 to <8 x i16>*
+  %12 = tail call <8 x i16> @llvm.masked.load.v8i16.p0v8i16(<8 x i16>* %11, i32 2, <8 x i1> %10, <8 x i16> zeroinitializer)
+  %13 = tail call fast <8 x half> @llvm.arm.mve.vcvt.fp.int.predicated.v8f16.v8i16.v8i1(<8 x i16> %12, i32 0, <8 x i1> %10, <8 x half> undef)
+  %14 = tail call fast <8 x half> @llvm.arm.mve.mul.predicated.v8f16.v8i1(<8 x half> %13, <8 x half> <half 0xH1800, half 0xH1800, half 0xH1800, half 0xH1800, half 0xH1800, half 0xH1800, half 0xH1800, half 0xH1800>, <8 x i1> %10, <8 x half> undef)
+  %15 = bitcast half* %pframef16.1 to <8 x half>*
+  tail call void @llvm.masked.store.v8f16.p0v8f16(<8 x half> %14, <8 x half>* %15, i32 2, <8 x i1> %10)
+  %add.ptr8 = getelementptr inbounds i16, i16* %px.0, i32 8
+  %add.ptr9 = getelementptr inbounds half, half* %pframef16.1, i32 8
+  %sub10 = add nsw i32 %blkCnt.1, -8
+  %cmp12 = icmp sgt i32 %blkCnt.1, 8
+  br i1 %cmp12, label %do.body6, label %do.end13
+
+do.end13:                                         ; preds = %do.body6
+  ret void
+}
+
+declare <8 x i1> @llvm.arm.mve.vctp16(i32)
+
+declare <8 x half> @llvm.masked.load.v8f16.p0v8f16(<8 x half>*, i32 immarg, <8 x i1>, <8 x half>)
+
+declare void @llvm.masked.store.v8f16.p0v8f16(<8 x half>, <8 x half>*, i32 immarg, <8 x i1>)
+
+declare <8 x i16> @llvm.masked.load.v8i16.p0v8i16(<8 x i16>*, i32 immarg, <8 x i1>, <8 x i16>)
+
+declare <8 x half> @llvm.arm.mve.vcvt.fp.int.predicated.v8f16.v8i16.v8i1(<8 x i16>, i32, <8 x i1>, <8 x half>)
+
+declare <8 x half> @llvm.arm.mve.mul.predicated.v8f16.v8i1(<8 x half>, <8 x half>, <8 x i1>, <8 x half>)