Don't sink gep operators through phi nodes if the result would require

more than one phi, since that leads to higher register pressure on
entry to the phi. This is especially problematic when the phi is in
a loop header, as it increases register pressure throughout the loop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81993 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 115 insertions, 1 deletions

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 47a02bd842..006d67b603 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -326,7 +326,7 @@ namespace {
     // instruction.  Instead, visit methods should return the value returned by
     // this function.
     Instruction *EraseInstFromFunction(Instruction &I) {
-      DEBUG(errs() << "IC: erase " << I);
+      DEBUG(errs() << "IC: erase " << I << '\n');
 
       assert(I.use_empty() && "Cannot erase instruction that is used!");
       // Make sure that we reprocess all operands now that we reduced their
@@ -10401,6 +10401,10 @@ Instruction *InstCombiner::FoldPHIArgGEPIntoPHI(PHINode &PN) {
   // This is true if all GEP bases are allocas and if all indices into them are
   // constants.
   bool AllBasePointersAreAllocas = true;
+
+  // We don't want to replace this phi if the replacement would require
+  // more than one phi.
+  bool NeededPhi = false;
   
   // Scan to see if all operands are the same opcode, all have one use, and all
   // kill their operands (i.e. the operands have one use).
@@ -10432,7 +10436,16 @@ Instruction *InstCombiner::FoldPHIArgGEPIntoPHI(PHINode &PN) {
       
       if (FirstInst->getOperand(op)->getType() !=GEP->getOperand(op)->getType())
         return 0;
+
+      // If we already needed a PHI for an earlier operand, and another operand
+      // also requires a PHI, we'd be introducing more PHIs than we're
+      // eliminating, which increases register pressure on entry to the PHI's
+      // block.
+      if (NeededPhi)
+        return 0;
+
       FixedOperands[op] = 0;  // Needs a PHI.
+      NeededPhi = true;
     }
   }
   
diff --git a/test/Transforms/InstCombine/phi-merge-gep.ll b/test/Transforms/InstCombine/phi-merge-gep.ll
new file mode 100644
index 0000000000..73c770e08c
--- /dev/null
+++ b/test/Transforms/InstCombine/phi-merge-gep.ll
@@ -0,0 +1,101 @@
+; RUN: opt < %s -S -instcombine > %t
+; RUN: grep {= getelementptr} %t | count 20
+; RUN: grep {= phi} %t | count 13
+
+; Don't push the geps through these phis; they have multiple uses!
+
+define void @foo(float* %Ar, float* %Ai, i64 %As, float* %Cr, float* %Ci, i64 %Cs, i64 %n) nounwind {
+entry:
+  %0 = getelementptr inbounds float* %Ar, i64 0   ; <float*> [#uses=1]
+  %1 = getelementptr inbounds float* %Ai, i64 0   ; <float*> [#uses=1]
+  %2 = mul i64 %n, %As                            ; <i64> [#uses=1]
+  %3 = getelementptr inbounds float* %Ar, i64 %2  ; <float*> [#uses=1]
+  %4 = mul i64 %n, %As                            ; <i64> [#uses=1]
+  %5 = getelementptr inbounds float* %Ai, i64 %4  ; <float*> [#uses=1]
+  %6 = mul i64 %n, 2                              ; <i64> [#uses=1]
+  %7 = mul i64 %6, %As                            ; <i64> [#uses=1]
+  %8 = getelementptr inbounds float* %Ar, i64 %7  ; <float*> [#uses=1]
+  %9 = mul i64 %n, 2                              ; <i64> [#uses=1]
+  %10 = mul i64 %9, %As                           ; <i64> [#uses=1]
+  %11 = getelementptr inbounds float* %Ai, i64 %10 ; <float*> [#uses=1]
+  %12 = getelementptr inbounds float* %Cr, i64 0  ; <float*> [#uses=1]
+  %13 = getelementptr inbounds float* %Ci, i64 0  ; <float*> [#uses=1]
+  %14 = mul i64 %n, %Cs                           ; <i64> [#uses=1]
+  %15 = getelementptr inbounds float* %Cr, i64 %14 ; <float*> [#uses=1]
+  %16 = mul i64 %n, %Cs                           ; <i64> [#uses=1]
+  %17 = getelementptr inbounds float* %Ci, i64 %16 ; <float*> [#uses=1]
+  %18 = mul i64 %n, 2                             ; <i64> [#uses=1]
+  %19 = mul i64 %18, %Cs                          ; <i64> [#uses=1]
+  %20 = getelementptr inbounds float* %Cr, i64 %19 ; <float*> [#uses=1]
+  %21 = mul i64 %n, 2                             ; <i64> [#uses=1]
+  %22 = mul i64 %21, %Cs                          ; <i64> [#uses=1]
+  %23 = getelementptr inbounds float* %Ci, i64 %22 ; <float*> [#uses=1]
+  br label %bb13
+
+bb:                                               ; preds = %bb13
+  %24 = load float* %A0r.0, align 4               ; <float> [#uses=1]
+  %25 = load float* %A0i.0, align 4               ; <float> [#uses=1]
+  %26 = load float* %A1r.0, align 4               ; <float> [#uses=2]
+  %27 = load float* %A1i.0, align 4               ; <float> [#uses=2]
+  %28 = load float* %A2r.0, align 4               ; <float> [#uses=2]
+  %29 = load float* %A2i.0, align 4               ; <float> [#uses=2]
+  %30 = fadd float %26, %28                       ; <float> [#uses=2]
+  %31 = fadd float %27, %29                       ; <float> [#uses=2]
+  %32 = fsub float %26, %28                       ; <float> [#uses=1]
+  %33 = fsub float %27, %29                       ; <float> [#uses=1]
+  %34 = fadd float %24, %30                       ; <float> [#uses=2]
+  %35 = fadd float %25, %31                       ; <float> [#uses=2]
+  %36 = fmul float %30, -1.500000e+00             ; <float> [#uses=1]
+  %37 = fmul float %31, -1.500000e+00             ; <float> [#uses=1]
+  %38 = fadd float %34, %36                       ; <float> [#uses=2]
+  %39 = fadd float %35, %37                       ; <float> [#uses=2]
+  %40 = fmul float %32, 0x3FEBB67AE0000000        ; <float> [#uses=2]
+  %41 = fmul float %33, 0x3FEBB67AE0000000        ; <float> [#uses=2]
+  %42 = fadd float %38, %41                       ; <float> [#uses=1]
+  %43 = fsub float %39, %40                       ; <float> [#uses=1]
+  %44 = fsub float %38, %41                       ; <float> [#uses=1]
+  %45 = fadd float %39, %40                       ; <float> [#uses=1]
+  store float %34, float* %C0r.0, align 4
+  store float %35, float* %C0i.0, align 4
+  store float %42, float* %C1r.0, align 4
+  store float %43, float* %C1i.0, align 4
+  store float %44, float* %C2r.0, align 4
+  store float %45, float* %C2i.0, align 4
+  %46 = getelementptr inbounds float* %A0r.0, i64 %As ; <float*> [#uses=1]
+  %47 = getelementptr inbounds float* %A0i.0, i64 %As ; <float*> [#uses=1]
+  %48 = getelementptr inbounds float* %A1r.0, i64 %As ; <float*> [#uses=1]
+  %49 = getelementptr inbounds float* %A1i.0, i64 %As ; <float*> [#uses=1]
+  %50 = getelementptr inbounds float* %A2r.0, i64 %As ; <float*> [#uses=1]
+  %51 = getelementptr inbounds float* %A2i.0, i64 %As ; <float*> [#uses=1]
+  %52 = getelementptr inbounds float* %C0r.0, i64 %Cs ; <float*> [#uses=1]
+  %53 = getelementptr inbounds float* %C0i.0, i64 %Cs ; <float*> [#uses=1]
+  %54 = getelementptr inbounds float* %C1r.0, i64 %Cs ; <float*> [#uses=1]
+  %55 = getelementptr inbounds float* %C1i.0, i64 %Cs ; <float*> [#uses=1]
+  %56 = getelementptr inbounds float* %C2r.0, i64 %Cs ; <float*> [#uses=1]
+  %57 = getelementptr inbounds float* %C2i.0, i64 %Cs ; <float*> [#uses=1]
+  %58 = add nsw i64 %i.0, 1                       ; <i64> [#uses=1]
+  br label %bb13
+
+bb13:                                             ; preds = %bb, %entry
+  %i.0 = phi i64 [ 0, %entry ], [ %58, %bb ]      ; <i64> [#uses=2]
+  %C2i.0 = phi float* [ %23, %entry ], [ %57, %bb ] ; <float*> [#uses=2]
+  %C2r.0 = phi float* [ %20, %entry ], [ %56, %bb ] ; <float*> [#uses=2]
+  %C1i.0 = phi float* [ %17, %entry ], [ %55, %bb ] ; <float*> [#uses=2]
+  %C1r.0 = phi float* [ %15, %entry ], [ %54, %bb ] ; <float*> [#uses=2]
+  %C0i.0 = phi float* [ %13, %entry ], [ %53, %bb ] ; <float*> [#uses=2]
+  %C0r.0 = phi float* [ %12, %entry ], [ %52, %bb ] ; <float*> [#uses=2]
+  %A2i.0 = phi float* [ %11, %entry ], [ %51, %bb ] ; <float*> [#uses=2]
+  %A2r.0 = phi float* [ %8, %entry ], [ %50, %bb ] ; <float*> [#uses=2]
+  %A1i.0 = phi float* [ %5, %entry ], [ %49, %bb ] ; <float*> [#uses=2]
+  %A1r.0 = phi float* [ %3, %entry ], [ %48, %bb ] ; <float*> [#uses=2]
+  %A0i.0 = phi float* [ %1, %entry ], [ %47, %bb ] ; <float*> [#uses=2]
+  %A0r.0 = phi float* [ %0, %entry ], [ %46, %bb ] ; <float*> [#uses=2]
+  %59 = icmp slt i64 %i.0, %n                     ; <i1> [#uses=1]
+  br i1 %59, label %bb, label %bb14
+
+bb14:                                             ; preds = %bb13
+  br label %return
+
+return:                                           ; preds = %bb14
+  ret void
+}