From 959b27677107891425e123121d46bfda0f398c9a Mon Sep 17 00:00:00 2001 From: Sanjay Patel Date: Tue, 28 Apr 2015 21:03:22 +0000 Subject: [PATCH] transform fadd chains to increase parallelism This is a compromise: with this simple patch, we should always handle a chain of exactly 3 operations optimally, but we're not generating the optimal balanced binary tree for a longer sequence. In general, this transform will reduce the dependency chain for a sequence of instructions using N operands from a worst case N-1 dependent operations to N/2 dependent operations. The optimal balanced binary tree would reduce the chain to log2(N). The trade-off for not dealing with longer sequences is: (1) we have less complexity in the compiler, (2) we avoid unknown compile-time blowup calculating a balanced tree, and (3) we don't need to worry about the increased register pressure required to parallelize longer sequences. It also seems unlikely that we would ever encounter really long strings of dependent ops like that in the wild, but I'm not sure how to verify that speculation. FWIW, I see no perf difference for test-suite running on btver2 (x86-64) with -ffast-math and this patch. We can extend this patch to cover other associative operations such as fmul, fmax, fmin, integer add, integer mul. This is a partial fix for: https://llvm.org/bugs/show_bug.cgi?id=17305 and if extended: https://llvm.org/bugs/show_bug.cgi?id=21768 https://llvm.org/bugs/show_bug.cgi?id=23116 The issue also came up in: http://reviews.llvm.org/D8941 Differential Revision: http://reviews.llvm.org/D9232 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236031 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 18 ++++++++++ test/CodeGen/X86/fp-fast.ll | 43 ++++++++++++++++++++++++ 2 files changed, 61 insertions(+) diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp index 629313c7120..66c1356152d 100644 --- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp +++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp @@ -7801,6 +7801,24 @@ SDValue DAGCombiner::visitFADD(SDNode *N) { N0.getOperand(0), DAG.getConstantFP(4.0, DL, VT)); } } + + // Canonicalize chains of adds to LHS to simplify the following transform. + if (N0.getOpcode() != ISD::FADD && N1.getOpcode() == ISD::FADD) + return DAG.getNode(ISD::FADD, SDLoc(N), VT, N1, N0); + + // Convert a chain of 3 dependent operations into 2 independent operations + // and 1 dependent operation: + // (fadd N0: (fadd N00: (fadd z, w), N01: y), N1: x) -> + // (fadd N00: (fadd z, w), (fadd N1: x, N01: y)) + if (N0.getOpcode() == ISD::FADD && N0.hasOneUse() && + N1.getOpcode() != ISD::FADD) { + SDValue N00 = N0.getOperand(0); + if (N00.getOpcode() == ISD::FADD) { + SDValue N01 = N0.getOperand(1); + SDValue NewAdd = DAG.getNode(ISD::FADD, SDLoc(N), VT, N1, N01); + return DAG.getNode(ISD::FADD, SDLoc(N), VT, N00, NewAdd); + } + } } // enable-unsafe-fp-math // FADD -> FMA combines: diff --git a/test/CodeGen/X86/fp-fast.ll b/test/CodeGen/X86/fp-fast.ll index 479f60d91f1..eb2ebd9119a 100644 --- a/test/CodeGen/X86/fp-fast.ll +++ b/test/CodeGen/X86/fp-fast.ll @@ -113,3 +113,46 @@ define float @test11(float %a) { %t2 = fadd float %a, %t1 ret float %t2 } + +; Verify that the first two adds are independent; the destination registers +; are used as source registers for the third add. + +define float @reassociate_adds1(float %a, float %b, float %c, float %d) { +; CHECK-LABEL: reassociate_adds1: +; CHECK: # BB#0: +; CHECK-NEXT: vaddss %xmm1, %xmm0, %xmm0 +; CHECK-NEXT: vaddss %xmm2, %xmm3, %xmm1 +; CHECK-NEXT: vaddss %xmm1, %xmm0, %xmm0 +; CHECK-NEXT: retq + %add0 = fadd float %a, %b + %add1 = fadd float %add0, %c + %add2 = fadd float %add1, %d + ret float %add2 +} + +define float @reassociate_adds2(float %a, float %b, float %c, float %d) { +; CHECK-LABEL: reassociate_adds2: +; CHECK: # BB#0: +; CHECK-NEXT: vaddss %xmm1, %xmm0, %xmm0 +; CHECK-NEXT: vaddss %xmm2, %xmm3, %xmm1 +; CHECK-NEXT: vaddss %xmm1, %xmm0, %xmm0 +; CHECK-NEXT: retq + %add0 = fadd float %a, %b + %add1 = fadd float %c, %add0 + %add2 = fadd float %add1, %d + ret float %add2 +} + +define float @reassociate_adds3(float %a, float %b, float %c, float %d) { +; CHECK-LABEL: reassociate_adds3: +; CHECK: # BB#0: +; CHECK-NEXT: vaddss %xmm1, %xmm0, %xmm0 +; CHECK-NEXT: vaddss %xmm2, %xmm3, %xmm1 +; CHECK-NEXT: vaddss %xmm1, %xmm0, %xmm0 +; CHECK-NEXT: retq + %add0 = fadd float %a, %b + %add1 = fadd float %add0, %c + %add2 = fadd float %d, %add1 + ret float %add2 +} + -- 2.34.1