ARM: don't try to hoist constant RHS out of a division.

Divisions by a constant can be converted into multiplies which are usually
cheaper, but this isn't possible if the constant gets separated (particularly
in loops). Fix this by telling ConstantHoisting that the immediate in a DIV is
cheap.

I considered making the check generic, but neither AArch64 (strangely) nor x86
showed any benefit on the tests I had.

llvm-svn: 266464

2 files changed, 16 insertions, 3 deletions

diff --git a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
index 932ec2d46da..aaec3107aec 100644
--- a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
@@ -47,6 +47,21 @@ int ARMTTIImpl::getIntImmCost(const APInt &Imm, Type *Ty) {
   return 3;
 }
 
+int ARMTTIImpl::getIntImmCost(unsigned Opcode, unsigned Idx, const APInt &Imm,
+                              Type *Ty) {
+  // Division by a constant can be turned into multiplication, but only if we
+  // know it's constant. So it's not so much that the immediate is cheap (it's
+  // not), but that the alternative is worse.
+  // FIXME: this is probably unneeded with GlobalISel.
+  if ((Opcode == Instruction::SDiv || Opcode == Instruction::UDiv ||
+       Opcode == Instruction::SRem || Opcode == Instruction::URem) &&
+      Idx == 1)
+    return 0;
+
+  return getIntImmCost(Imm, Ty);
+}
+
+
 int ARMTTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst, Type *Src) {
   int ISD = TLI->InstructionOpcodeToISD(Opcode);
   assert(ISD && "Invalid opcode");
diff --git a/llvm/lib/Target/ARM/ARMTargetTransformInfo.h b/llvm/lib/Target/ARM/ARMTargetTransformInfo.h
index 0fe964f36de..e07174eb40b 100644
--- a/llvm/lib/Target/ARM/ARMTargetTransformInfo.h
+++ b/llvm/lib/Target/ARM/ARMTargetTransformInfo.h
@@ -64,9 +64,7 @@ public:
   using BaseT::getIntImmCost;
   int getIntImmCost(const APInt &Imm, Type *Ty);
 
-  int getIntImmCost(unsigned Opcode, unsigned Idx, const APInt &Imm, Type *Ty) {
-    return getIntImmCost(Imm, Ty);
-  }
+  int getIntImmCost(unsigned Opcode, unsigned Idx, const APInt &Imm, Type *Ty);
 
   /// @}