[CodeExtractor] Fix sinking of allocas with multiple bitcast uses (PR42451)

[vsk]

Summary

An alloca which can be sunk into the extraction region may have more
than one bitcast use. Move these uses along with the alloca to prevent
use-before-def.

Testing: check-llvm, stage2 build of clang

Fixes llvm.org/PR42451.

[davidxl]

Is it possible to split the patch into a NFC refactoring and one functional change?

llvm/lib/Transforms/Utils/CodeExtractor.cpp

413 ↗

(On Diff #208858)

Add a comment for the function.

[vsk]

• Rebase on an NFC refactoring: D64467

[davidxl]

lgtm

llvm/trunk/lib/Transforms/Utils/CodeExtractor.cpp

for (Instruction &II : BB) {
           break;
         }
       }
 
       // Either no bitcasts reference the alloca or there are unknown uses.
       if (Bitcasts.empty())
         continue;
 
-      Instruction *BitcastAddr = Bitcasts.back();
-      const LifetimeMarkerInfo &LMI = BitcastLifetimeInfo.back();
+      LLVM_DEBUG(dbgs() << "Sinking alloca (via bitcast): " << *AI << "\n");
+      SinkCands.insert(AI);
+      for (unsigned I = 0, E = Bitcasts.size(); I != E; ++I) {
+        Instruction *BitcastAddr = Bitcasts[I];
+        const LifetimeMarkerInfo &LMI = BitcastLifetimeInfo[I];
         assert(LMI.LifeStart &&
                "Unsafe to sink bitcast without lifetime markers");
         moveOrIgnoreLifetimeMarkers(LMI);
         if (!definedInRegion(Blocks, BitcastAddr)) {
           LLVM_DEBUG(dbgs() << "Sinking bitcast-of-alloca: " << *BitcastAddr
                             << "\n");
           SinkCands.insert(BitcastAddr);
         }
-      LLVM_DEBUG(dbgs() << "Sinking alloca (via bitcast): " << *AI << "\n");
-      SinkCands.insert(AI);
+      }
     }
   }
 }
 
 void CodeExtractor::findInputsOutputs(ValueSet &Inputs, ValueSet &Outputs,
                                       const ValueSet &SinkCands) const {
   for (BasicBlock *BB : Blocks) {
     // If a used value is defined outside the region, it's an input.  If an
Function *CodeExtractor::extractCodeRegion() {
 
   findAllocas(SinkingCands, HoistingCands, CommonExit);
   assert(HoistingCands.empty() || CommonExit);
 
   // Find inputs to, outputs from the code region.
   findInputsOutputs(inputs, outputs, SinkingCands);
 
   // Now sink all instructions which only have non-phi uses inside the region.
-  for (auto *II : SinkingCands)
-    cast<Instruction>(II)->moveBefore(*newFuncRoot,
-                                      newFuncRoot->getFirstInsertionPt());
+  // Group the allocas at the start of the block, so that any bitcast uses of
+  // the allocas are well-defined.
+  AllocaInst *FirstSunkAlloca = nullptr;
+  for (auto *II : SinkingCands) {
+    if (auto *AI = dyn_cast<AllocaInst>(II)) {
+      AI->moveBefore(*newFuncRoot, newFuncRoot->getFirstInsertionPt());
+      if (!FirstSunkAlloca)
+        FirstSunkAlloca = AI;
+    }
+  }
+  assert((SinkingCands.empty() || FirstSunkAlloca) &&
+         "Did not expect a sink candidate without any allocas");
+  for (auto *II : SinkingCands) {
+    if (!isa<AllocaInst>(II)) {
+      cast<Instruction>(II)->moveAfter(FirstSunkAlloca);
+    }
+  }
 
   if (!HoistingCands.empty()) {
     auto *HoistToBlock = findOrCreateBlockForHoisting(CommonExit);
     Instruction *TI = HoistToBlock->getTerminator();
     for (auto *II : HoistingCands)
       cast<Instruction>(II)->moveBefore(TI);
   }
 

llvm/trunk/test/Transforms/CodeExtractor/live_shrink_multiple.ll

 bb:
   tail call void @_Z3foov()
   ret void
 }
 
 ; CHECK-LABEL: define internal void @_Z3foov.1.
 ; CHECK: newFuncRoot:
 ; CHECK-NEXT:  alloca 
+; CHECK-NEXT:  alloca
 ; CHECK-NEXT:  bitcast 
 ; CHECK-NEXT:  call void @llvm.lifetime.start.p0i8
-; CHECK-NEXT:  alloca
 ; CHECK-NEXT:  bitcast 
 ; CHECK-NEXT:  call void @llvm.lifetime.start.p0i8
 ; CHECK:  call void @llvm.lifetime.end.p0i8
 ; CHECK-NEXT:  call void @llvm.lifetime.end.p0i8
 ; CHECK-NEXT:  br label {{.*}}exitStub
 
 
 !llvm.module.flags = !{!0}
 !llvm.ident = !{!1}
 
 !0 = !{i32 1, !"wchar_size", i32 4}
 !1 = !{!"clang version 5.0.0 (trunk 304489)"}
 !2 = !{!3, !3, i64 0}
 !3 = !{!"int", !4, i64 0}
 !4 = !{!"omnipotent char", !5, i64 0}
 !5 = !{!"Simple C++ TBAA"}

llvm/trunk/test/Transforms/HotColdSplit/sink-multiple-bitcasts-of-allocas-pr42451.ll

+; RUN: opt -hotcoldsplit -hotcoldsplit-threshold=-1 -S < %s | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.14.0"
+
+@c = common global i32 0, align 4
+@h = common global i32 0, align 4
+
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #1
+
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #1
+
+declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1 immarg) #1
+
+declare i32* @m()
+
+; CHECK-LABEL: define void @main()
+; CHECK-NEXT:   %.sroa.4.i = alloca [20 x i8], align 2
+; CHECK-NEXT:   %.sroa.5.i = alloca [6 x i8], align 8
+; CHECK-NEXT:   %1 = bitcast [6 x i8]* %.sroa.5.i to i8*
+
+define void @main() #0 {
+  %.sroa.4.i = alloca [20 x i8], align 2
+  %.sroa.5.i = alloca [6 x i8], align 8
+  %1 = bitcast [6 x i8]* %.sroa.5.i to i8*
+  %2 = load i32, i32* @h, align 4, !tbaa !4
+  %3 = icmp ne i32 %2, 0
+  br i1 %3, label %12, label %4
+
+4:                                                ; preds = %0
+  %5 = call i32* @m() #3
+  %.sroa.4.0..sroa_idx21.i = getelementptr inbounds [20 x i8], [20 x i8]* %.sroa.4.i, i64 0, i64 0
+  call void @llvm.lifetime.start.p0i8(i64 20, i8* %.sroa.4.0..sroa_idx21.i) #3
+  %.sroa.5.0..sroa_idx16.i = getelementptr inbounds [6 x i8], [6 x i8]* %.sroa.5.i, i64 0, i64 0
+  call void @llvm.lifetime.start.p0i8(i64 6, i8* %.sroa.5.0..sroa_idx16.i) #3
+  call void @llvm.memset.p0i8.i64(i8* align 2 %.sroa.4.0..sroa_idx21.i, i8 0, i64 20, i1 false) #3
+  call void @llvm.memset.p0i8.i64(i8* align 8 %.sroa.5.0..sroa_idx16.i, i8 0, i64 6, i1 false) #3
+  %6 = load i32, i32* @c, align 4, !tbaa !4
+  %7 = trunc i32 %6 to i16
+  call void @llvm.lifetime.end.p0i8(i64 20, i8* %.sroa.4.0..sroa_idx21.i) #3
+  call void @llvm.lifetime.end.p0i8(i64 6, i8* %.sroa.5.0..sroa_idx16.i) #3
+  call void @llvm.lifetime.start.p0i8(i64 6, i8* %1) #3
+  call void @llvm.memset.p0i8.i64(i8* align 1 %1, i8 3, i64 6, i1 false)
+  br label %8
+
+8:                                                ; preds = %8, %4
+  %.0.i = phi i32 [ 0, %4 ], [ %10, %8 ]
+  %9 = sext i32 %.0.i to i64
+  %10 = add nsw i32 %.0.i, 1
+  %11 = icmp slt i32 %10, 6
+  br i1 %11, label %8, label %l.exit
+
+l.exit:                                           ; preds = %8
+  call void @llvm.lifetime.end.p0i8(i64 6, i8* %1) #3
+  br label %12
+
+12:                                               ; preds = %l.exit, %0
+  %13 = phi i1 [ true, %0 ], [ true, %l.exit ]
+  ret void
+}
+
+attributes #0 = { cold }
+
+!llvm.module.flags = !{!0, !1, !2}
+!llvm.ident = !{!3}
+
+!0 = !{i32 2, !"SDK Version", [2 x i32] [i32 10, i32 14]}
+!1 = !{i32 1, !"wchar_size", i32 4}
+!2 = !{i32 7, !"PIC Level", i32 2}
+!3 = !{!"Apple clang version 11.0.0 (clang-1100.0.20.17)"}
+!4 = !{!5, !5, i64 0}
+!5 = !{!"int", !6, i64 0}
+!6 = !{!"omnipotent char", !7, i64 0}
+!7 = !{!"Simple C/C++ TBAA"}