Do actual DCE in LoopUnroll

LoopUnroll does a limited DCE pass after unrolling, but if you have a chain of dead instructions, it only deletes the last one. Improve the code to recursively delete all trivially dead instructions.

Differential Revision: https://reviews.llvm.org/D102511

llvm/lib/Transforms/Utils/LoopUnroll.cpp

@@ -220,10 +220,10 @@ void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
     }
   }
 
-  // At this point, the code is well formed.  We now do a quick sweep over the
-  // inserted code, doing constant propagation and dead code elimination as we
-  // go.
+  // At this point, the code is well formed.  Perform constprop, instsimplify,
+  // and dce.
   const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
+  SmallVector<WeakTrackingVH, 16> DeadInsts;
   for (BasicBlock *BB : L->getBlocks()) {
     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
       Instruction *Inst = &*I++;
@@ -232,14 +232,15 @@ void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
         if (LI->replacementPreservesLCSSAForm(Inst, V))
           Inst->replaceAllUsesWith(V);
       if (isInstructionTriviallyDead(Inst))
-        BB->getInstList().erase(Inst);
+        DeadInsts.emplace_back(Inst);
     }
   }
 
-  // TODO: after peeling or unrolling, previously loop variant conditions are
-  // likely to fold to constants, eagerly propagating those here will require
-  // fewer cleanup passes to be run.  Alternatively, a LoopEarlyCSE might be
-  // appropriate.
+  while (!DeadInsts.empty()) {
+    Value *V = DeadInsts.pop_back_val();
+    if (Instruction *Inst = dyn_cast_or_null<Instruction>(V))
+      RecursivelyDeleteTriviallyDeadInstructions(Inst);
+  }
 }
 
 /// Unroll the given loop by Count. The loop must be in LCSSA form.  Unrolling

llvm/test/Transforms/LoopUnroll/nonlatchcondbr.ll

@@ -40,7 +40,6 @@ define void @test1(i32* noalias %A) {
 ; CHECK:       for.body.3:
 ; CHECK-NEXT:    br i1 false, label [[FOR_BODY_FOR_BODY_CRIT_EDGE_3:%.*]], label [[FOR_END:%.*]]
 ; CHECK:       for.body.for.body_crit_edge.3:
-; CHECK-NEXT:    [[ARRAYIDX_PHI_TRANS_INSERT_3:%.*]] = getelementptr inbounds i32, i32* [[A]], i64 4
 ; CHECK-NEXT:    unreachable
 ;
 entry:
@@ -124,7 +123,7 @@ define void @test2(i32* noalias %A) {
 ; CHECK:       for.body.for.body_crit_edge.3:
 ; CHECK-NEXT:    [[ARRAYIDX_PHI_TRANS_INSERT_3:%.*]] = getelementptr inbounds i32, i32* [[A]], i64 [[INC_3]]
 ; CHECK-NEXT:    [[DOTPRE_3]] = load i32, i32* [[ARRAYIDX_PHI_TRANS_INSERT_3]], align 4
-; CHECK-NEXT:    br label [[FOR_HEADER]], !llvm.loop !0
+; CHECK-NEXT:    br label [[FOR_HEADER]], !llvm.loop [[LOOP0:![0-9]+]]
 ;
 entry:
   br i1 true, label %for.preheader, label %for.end
@@ -202,7 +201,7 @@ define void @test3(i32* noalias %A, i1 %cond) {
 ; CHECK:       for.body.for.body_crit_edge.3:
 ; CHECK-NEXT:    [[ARRAYIDX_PHI_TRANS_INSERT_3:%.*]] = getelementptr inbounds i32, i32* [[A]], i64 [[INC_3]]
 ; CHECK-NEXT:    [[DOTPRE_3]] = load i32, i32* [[ARRAYIDX_PHI_TRANS_INSERT_3]], align 4
-; CHECK-NEXT:    br label [[FOR_HEADER]], !llvm.loop !2
+; CHECK-NEXT:    br label [[FOR_HEADER]], !llvm.loop [[LOOP2:![0-9]+]]
 ;
 entry:
   %0 = load i32, i32* %A, align 4

llvm/test/Transforms/LoopUnroll/optsize-loop-size.ll

@@ -12,7 +12,6 @@ define i32 @test(i32 %a, i32 %b, i32 %c) optsize {
 ; CHECK-NEXT:    store i32 [[B:%.*]], i32* [[ARRAYINIT_ELEMENT]], align 4
 ; CHECK-NEXT:    [[ARRAYINIT_ELEMENT1:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[REF_TMP]], i64 0, i64 2
 ; CHECK-NEXT:    store i32 [[C:%.*]], i32* [[ARRAYINIT_ELEMENT1]], align 4
-; CHECK-NEXT:    [[ADD_PTR_I_I:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[REF_TMP]], i64 0, i64 3
 ; CHECK-NEXT:    [[CMP_I_I_I3:%.*]] = icmp slt i32 [[A]], [[B]]
 ; CHECK-NEXT:    [[SPEC_SELECT_I_I4:%.*]] = select i1 [[CMP_I_I_I3]], i32* [[ARRAYINIT_ELEMENT]], i32* [[ARRAYINIT_BEGIN]]
 ; CHECK-NEXT:    [[INCDEC_PTR_I_I5:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[REF_TMP]], i64 0, i64 2
@@ -22,7 +21,6 @@ define i32 @test(i32 %a, i32 %b, i32 %c) optsize {
 ; CHECK-NEXT:    [[DOTPRE2:%.*]] = load i32, i32* [[INCDEC_PTR_I_I5]], align 4
 ; CHECK-NEXT:    [[CMP_I_I_I:%.*]] = icmp slt i32 [[DOTPRE]], [[DOTPRE2]]
 ; CHECK-NEXT:    [[SPEC_SELECT_I_I:%.*]] = select i1 [[CMP_I_I_I]], i32* [[INCDEC_PTR_I_I5]], i32* [[SPEC_SELECT_I_I4]]
-; CHECK-NEXT:    [[INCDEC_PTR_I_I:%.*]] = getelementptr inbounds i32, i32* [[INCDEC_PTR_I_I5]], i64 1
 ; CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* [[SPEC_SELECT_I_I]], align 4
 ; CHECK-NEXT:    ret i32 [[TMP1]]
 ;

llvm/test/Transforms/LoopUnroll/pr45939-peel-count-and-complete-unroll.ll

@@ -163,7 +163,6 @@ define void @test1() {
 ; PEEL8-NEXT:    [[ARRAYIDX_7:%.*]] = getelementptr inbounds [8 x i32], [8 x i32]* @a, i64 0, i64 [[INDVARS_IV_NEXT_6]]
 ; PEEL8-NEXT:    [[TMP15:%.*]] = trunc i64 [[INDVARS_IV_NEXT_6]] to i32
 ; PEEL8-NEXT:    store i32 [[TMP15]], i32* [[ARRAYIDX_7]], align 4
-; PEEL8-NEXT:    [[INDVARS_IV_NEXT_7:%.*]] = add nuw nsw i64 [[INDVARS_IV_NEXT_6]], 1
 ; PEEL8-NEXT:    br label [[FOR_EXIT]]
 ; PEEL8:       for.exit:
 ; PEEL8-NEXT:    ret void
@@ -206,7 +205,7 @@ define void @test1() {
 ; PEEL2UNROLL2-NEXT:    store i32 [[TMP3]], i32* [[ARRAYIDX_1]], align 4
 ; PEEL2UNROLL2-NEXT:    [[INDVARS_IV_NEXT_1]] = add nuw nsw i64 [[INDVARS_IV_NEXT]], 1
 ; PEEL2UNROLL2-NEXT:    [[EXITCOND_1:%.*]] = icmp ne i64 [[INDVARS_IV_NEXT_1]], 8
-; PEEL2UNROLL2-NEXT:    br i1 [[EXITCOND_1]], label [[FOR_BODY]], label [[FOR_EXIT_LOOPEXIT:%.*]], !llvm.loop !0
+; PEEL2UNROLL2-NEXT:    br i1 [[EXITCOND_1]], label [[FOR_BODY]], label [[FOR_EXIT_LOOPEXIT:%.*]], !llvm.loop [[LOOP0:![0-9]+]]
 ; PEEL2UNROLL2:       for.exit.loopexit:
 ; PEEL2UNROLL2-NEXT:    br label [[FOR_EXIT]]
 ; PEEL2UNROLL2:       for.exit:

llvm/test/Transforms/LoopUnroll/scevunroll.ll

@@ -10,7 +10,6 @@
 define i32 @sansCanonical(i32* %base) nounwind {
 ; CHECK-LABEL: @sansCanonical(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[ZEXT:%.*]] = zext i32 0 to i64
 ; CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
 ; CHECK:       while.body:
 ; CHECK-NEXT:    [[ADR:%.*]] = getelementptr inbounds i32, i32* [[BASE:%.*]], i64 9
@@ -39,7 +38,6 @@ define i32 @sansCanonical(i32* %base) nounwind {
 ; CHECK-NEXT:    [[ADR_8:%.*]] = getelementptr inbounds i32, i32* [[BASE]], i64 1
 ; CHECK-NEXT:    [[TMP_8:%.*]] = load i32, i32* [[ADR_8]], align 8
 ; CHECK-NEXT:    [[SUM_NEXT_8:%.*]] = add i32 [[SUM_NEXT_7]], [[TMP_8]]
-; CHECK-NEXT:    [[TMP_9:%.*]] = load i32, i32* [[BASE]], align 8
 ; CHECK-NEXT:    ret i32 [[SUM_NEXT_8]]
 ;
 entry:

llvm/test/Transforms/LoopUnroll/unroll-header-exiting-with-phis.ll

@@ -10,23 +10,18 @@ define i16 @full_unroll(i16* %A) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[FOR_COND:%.*]]
 ; CHECK:       for.cond:
-; CHECK-NEXT:    [[TMP2:%.*]] = load i16, i16* [[A:%.*]], align 2
 ; CHECK-NEXT:    br label [[FOR_COND_CLEANUP3:%.*]]
 ; CHECK:       for.cond.cleanup:
 ; CHECK-NEXT:    [[DOTLCSSA10_LCSSA:%.*]] = phi i16 [ [[TMP2_2:%.*]], [[FOR_COND_CLEANUP3_2:%.*]] ]
 ; CHECK-NEXT:    [[TMP3:%.*]] = call i16 @func(i16 [[DOTLCSSA10_LCSSA]])
 ; CHECK-NEXT:    ret i16 0
 ; CHECK:       for.cond.cleanup3:
-; CHECK-NEXT:    [[PTR_1:%.*]] = getelementptr inbounds i16, i16* [[A]], i64 1
-; CHECK-NEXT:    [[TMP2_1:%.*]] = load i16, i16* [[PTR_1]], align 2
 ; CHECK-NEXT:    br label [[FOR_COND_CLEANUP3_1:%.*]]
 ; CHECK:       for.cond.cleanup3.1:
-; CHECK-NEXT:    [[PTR_2:%.*]] = getelementptr inbounds i16, i16* [[A]], i64 2
+; CHECK-NEXT:    [[PTR_2:%.*]] = getelementptr inbounds i16, i16* [[A:%.*]], i64 2
 ; CHECK-NEXT:    [[TMP2_2]] = load i16, i16* [[PTR_2]], align 2
 ; CHECK-NEXT:    br label [[FOR_COND_CLEANUP3_2]]
 ; CHECK:       for.cond.cleanup3.2:
-; CHECK-NEXT:    [[PTR_3:%.*]] = getelementptr inbounds i16, i16* [[A]], i64 3
-; CHECK-NEXT:    [[TMP2_3:%.*]] = load i16, i16* [[PTR_3]], align 2
 ; CHECK-NEXT:    br i1 false, label [[FOR_COND_CLEANUP3_3:%.*]], label [[FOR_COND_CLEANUP:%.*]]
 ; CHECK:       for.cond.cleanup3.3:
 ; CHECK-NEXT:    unreachable
@@ -59,22 +54,18 @@ define i16 @partial_unroll(i16* %A) {
 ; CHECK-NEXT:    br label [[FOR_COND:%.*]]
 ; CHECK:       for.cond:
 ; CHECK-NEXT:    [[I_0:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[INC9_2:%.*]], [[FOR_COND_CLEANUP3_2:%.*]] ]
-; CHECK-NEXT:    [[PTR:%.*]] = getelementptr inbounds i16, i16* [[A:%.*]], i64 [[I_0]]
-; CHECK-NEXT:    [[TMP2:%.*]] = load i16, i16* [[PTR]], align 2
 ; CHECK-NEXT:    br label [[FOR_COND_CLEANUP3:%.*]]
 ; CHECK:       for.cond.cleanup:
 ; CHECK-NEXT:    [[DOTLCSSA10_LCSSA:%.*]] = phi i16 [ [[TMP2_1:%.*]], [[FOR_COND_CLEANUP3_1:%.*]] ]
 ; CHECK-NEXT:    [[TMP3:%.*]] = call i16 @func(i16 [[DOTLCSSA10_LCSSA]])
 ; CHECK-NEXT:    ret i16 0
 ; CHECK:       for.cond.cleanup3:
 ; CHECK-NEXT:    [[INC9:%.*]] = add nuw nsw i64 [[I_0]], 1
-; CHECK-NEXT:    [[PTR_1:%.*]] = getelementptr inbounds i16, i16* [[A]], i64 [[INC9]]
+; CHECK-NEXT:    [[PTR_1:%.*]] = getelementptr inbounds i16, i16* [[A:%.*]], i64 [[INC9]]
 ; CHECK-NEXT:    [[TMP2_1]] = load i16, i16* [[PTR_1]], align 2
 ; CHECK-NEXT:    br label [[FOR_COND_CLEANUP3_1]]
 ; CHECK:       for.cond.cleanup3.1:
 ; CHECK-NEXT:    [[INC9_1:%.*]] = add nuw nsw i64 [[INC9]], 1
-; CHECK-NEXT:    [[PTR_2:%.*]] = getelementptr inbounds i16, i16* [[A]], i64 [[INC9_1]]
-; CHECK-NEXT:    [[TMP2_2:%.*]] = load i16, i16* [[PTR_2]], align 2
 ; CHECK-NEXT:    [[CMP_2:%.*]] = icmp ult i64 [[INC9_1]], 200
 ; CHECK-NEXT:    br i1 [[CMP_2]], label [[FOR_COND_CLEANUP3_2]], label [[FOR_COND_CLEANUP:%.*]]
 ; CHECK:       for.cond.cleanup3.2:

llvm/test/Transforms/LoopUnroll/unroll-unconditional-latch.ll

@@ -68,7 +68,6 @@ define double @test_with_lcssa(double %arg1, double* %arg2) {
 ; CHECK-NEXT:    [[RES_LCSSA:%.*]] = phi double [ [[RES_1]], [[LOOP_LATCH]] ]
 ; CHECK-NEXT:    ret double [[RES_LCSSA]]
 ; CHECK:       loop.latch.1:
-; CHECK-NEXT:    [[PTR_1:%.*]] = getelementptr inbounds double, double* [[ARG2]], i64 2
 ; CHECK-NEXT:    unreachable
 ;
 

llvm/test/Transforms/LoopUnrollAndJam/unroll-and-jam.ll

@@ -141,8 +141,6 @@ target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
 ; CHECK-NEXT:    [[ADD_LCSSA_EPIL_2:%.*]] = phi i32 [ [[ADD_EPIL_2]], [[FOR_INNER_EPIL_2]] ]
 ; CHECK-NEXT:    [[ARRAYIDX6_EPIL_2:%.*]] = getelementptr inbounds i32, i32* [[A]], i32 [[ADD8_EPIL_1]]
 ; CHECK-NEXT:    store i32 [[ADD_LCSSA_EPIL_2]], i32* [[ARRAYIDX6_EPIL_2]], align 4, !tbaa !0
-; CHECK-NEXT:    [[ADD8_EPIL_2:%.*]] = add nuw i32 [[ADD8_EPIL_1]], 1
-; CHECK-NEXT:    [[EPIL_ITER_SUB_2:%.*]] = sub i32 [[EPIL_ITER_SUB_1]], 1
 ; CHECK-NEXT:    br label [[FOR_END_LOOPEXIT_EPILOG_LCSSA]]
 define void @test1(i32 %I, i32 %J, i32* noalias nocapture %A, i32* noalias nocapture readonly %B) #0 {
 entry: