[SystemZ::TTI] Return zero cost for ICmp that becomes Load And Test.

A loaded value with multiple users compared with 0 will become a load and
test single instruction. The load is not folded in this case (multiple
users), but the compare instruction is eliminated.

This patch returns 0 cost for the icmp in these cases.

Review: Ulrich Weigand
https://reviews.llvm.org/D55111

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348141 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/SystemZ/SystemZTargetTransformInfo.cpp

@@ -839,6 +839,16 @@ int SystemZTTIImpl::getCmpSelInstrCost(unsigned Opcode, Type *ValTy,
   else { // Scalar
     switch (Opcode) {
     case Instruction::ICmp: {
+      // A loaded value compared with 0 with multiple users becomes Load and
+      // Test. The load is then not foldable, so return 0 cost for the ICmp.
+      unsigned ScalarBits = ValTy->getScalarSizeInBits();
+      if (I != nullptr && ScalarBits >= 32)
+        if (LoadInst *Ld = dyn_cast<LoadInst>(I->getOperand(0)))
+          if (const ConstantInt *C = dyn_cast<ConstantInt>(I->getOperand(1)))
+            if (!Ld->hasOneUse() && Ld->getParent() == I->getParent() &&
+                C->getZExtValue() == 0)
+              return 0;
+
       unsigned Cost = 1;
       if (ValTy->isIntegerTy() && ValTy->getScalarSizeInBits() <= 16)
         Cost += (I != nullptr ? getOperandsExtensionCost(I) : 2);

test/Analysis/CostModel/SystemZ/load-and-test.ll

@@ -0,0 +1,25 @@
+; RUN: opt < %s -cost-model -analyze -mtriple=systemz-unknown -mcpu=z13 | FileCheck %s
+;
+; Test that load and test results in 0 cost for the compare.
+
+define i64 @fun0(i64* %Src, i64 %Arg) {
+  %Ld1 = load i64, i64* %Src
+  %Cmp = icmp eq i64 %Ld1, 0
+  %S   = select i1 %Cmp, i64 %Arg, i64 %Ld1
+  ret i64 %S
+; CHECK: Printing analysis 'Cost Model Analysis' for function 'fun0':
+; CHECK: Cost Model: Found an estimated cost of 1 for instruction:   %Ld1 = load i64, i64* %Src
+; CHECK: Cost Model: Found an estimated cost of 0 for instruction:   %Cmp = icmp eq i64 %Ld1, 0
+; CHECK: Cost Model: Found an estimated cost of 1 for instruction:   %S = select
+}
+
+define i32 @fun1(i32* %Src, i32 %Arg) {
+  %Ld1 = load i32, i32* %Src
+  %Cmp = icmp eq i32 %Ld1, 0
+  %S   = select i1 %Cmp, i32 %Arg, i32 %Ld1
+  ret i32 %S
+; CHECK: Printing analysis 'Cost Model Analysis' for function 'fun1':
+; CHECK: Cost Model: Found an estimated cost of 1 for instruction:   %Ld1 = load i32, i32* %Src
+; CHECK: Cost Model: Found an estimated cost of 0 for instruction:   %Cmp = icmp eq i32 %Ld1, 0
+; CHECK: Cost Model: Found an estimated cost of 1 for instruction:   %S = select
+}