[llvm] r184872 - X86 cost model: Vectorizing integer division is a bad idea

[Arnold Schwaighofer]

Author: arnolds
Date: Tue Jun 25 14:14:09 2013
New Revision: 184872

URL: http://llvm.org/viewvc/llvm-project?rev=184872&view=rev
Log:
X86 cost model: Vectorizing integer division is a bad idea

radar://14057959

Added:
    llvm/trunk/test/Analysis/CostModel/X86/div.ll
Modified:
    llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp

Modified: llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp?rev=184872&r1=184871&r2=184872&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp Tue Jun 25 14:14:09 2013
@@ -196,6 +196,16 @@ unsigned X86TTI::getArithmeticInstrCost(
     { ISD::SRA,  MVT::v32i8,  32*10 }, // Scalarized.
     { ISD::SRA,  MVT::v16i16,  16*10 }, // Scalarized.
     { ISD::SRA,  MVT::v4i64,  4*10 }, // Scalarized.
+
+    // Vectorizing division is a bad idea. See the SSE2 table for more comments.
+    { ISD::SDIV,  MVT::v32i8,  32*20 },
+    { ISD::SDIV,  MVT::v16i16, 16*20 },
+    { ISD::SDIV,  MVT::v8i32,  8*20 },
+    { ISD::SDIV,  MVT::v4i64,  4*20 },
+    { ISD::UDIV,  MVT::v32i8,  32*20 },
+    { ISD::UDIV,  MVT::v16i16, 16*20 },
+    { ISD::UDIV,  MVT::v8i32,  8*20 },
+    { ISD::UDIV,  MVT::v4i64,  4*20 },
   };
 
   // Look for AVX2 lowering tricks.
@@ -258,6 +268,21 @@ unsigned X86TTI::getArithmeticInstrCost(
     { ISD::SRA,  MVT::v8i16,  8*10 }, // Scalarized.
     { ISD::SRA,  MVT::v4i32,  4*10 }, // Scalarized.
     { ISD::SRA,  MVT::v2i64,  2*10 }, // Scalarized.
+
+    // It is not a good idea to vectorize division. We have to scalarize it and
+    // in the process we will often end up having to spilling regular
+    // registers. The overhead of division is going to dominate most kernels
+    // anyways so try hard to prevent vectorization of division - it is
+    // generally a bad idea. Assume somewhat arbitrarily that we have to be able
+    // to hide "20 cycles" for each lane.
+    { ISD::SDIV,  MVT::v16i8,  16*20 },
+    { ISD::SDIV,  MVT::v8i16,  8*20 },
+    { ISD::SDIV,  MVT::v4i32,  4*20 },
+    { ISD::SDIV,  MVT::v2i64,  2*20 },
+    { ISD::UDIV,  MVT::v16i8,  16*20 },
+    { ISD::UDIV,  MVT::v8i16,  8*20 },
+    { ISD::UDIV,  MVT::v4i32,  4*20 },
+    { ISD::UDIV,  MVT::v2i64,  2*20 },
   };
 
   if (ST->hasSSE2()) {

Added: llvm/trunk/test/Analysis/CostModel/X86/div.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/CostModel/X86/div.ll?rev=184872&view=auto
==============================================================================
--- llvm/trunk/test/Analysis/CostModel/X86/div.ll (added)
+++ llvm/trunk/test/Analysis/CostModel/X86/div.ll Tue Jun 25 14:14:09 2013
@@ -0,0 +1,32 @@
+; RUN: opt -mtriple=x86_64-apple-darwin -mcpu=core2 -cost-model -analyze < %s | FileCheck --check-prefix=SSE2 %s
+; RUN: opt -mtriple=x86_64-apple-darwin -mcpu=core-avx2 -cost-model -analyze < %s | FileCheck --check-prefix=AVX2 %s
+
+
+define void @div_sse() {
+  ; SSE2: div_sse
+  ; SSE2: cost of 320 {{.*}} sdiv
+  %a0 = sdiv <16 x i8> undef, undef
+  ; SSE2: cost of 160 {{.*}} sdiv
+  %a1 = sdiv <8 x i16> undef, undef
+  ; SSE2: cost of 80 {{.*}} sdiv
+  %a2 = sdiv <4 x i32> undef, undef
+  ; SSE2: cost of 40 {{.*}} sdiv
+  %a3 = sdiv <2 x i32> undef, undef
+  ret void
+}
+; SSE2: div_avx
+
+define void @div_avx() {
+  ; AVX2: div_avx
+  ; AVX2: cost of 640 {{.*}} sdiv
+  %a0 = sdiv <32 x i8> undef, undef
+  ; AVX2: cost of 320 {{.*}} sdiv
+  %a1 = sdiv <16 x i16> undef, undef
+  ; AVX2: cost of 160 {{.*}} sdiv
+  %a2 = sdiv <8 x i32> undef, undef
+  ; AVX2: cost of 80 {{.*}} sdiv
+  %a3 = sdiv <4 x i32> undef, undef
+  ret void
+}
+
+