[SystemZ] Remove incorrect areInlineCompatible hook (llvm#152494)

This reverts llvm#132976.

The PR incorrectly claimed that this makes inlining more liberal,
referencing the string comparison in TargetTransformInfoImpl.h.

However, the implementation that actually applies is the one in
BasicTTIImpl.h, which performs a feature subset comparison. As such,
this regressed inlining, most concerningly of functions without +vector
into functions with +vector.

Revert the change to restore the previous behavior.

(cherry picked from commit 18e4f77)

Author: nikic

llvm/lib/Target/SystemZ/SystemZTargetTransformInfo.cpp

@@ -436,20 +436,6 @@ bool SystemZTTIImpl::isLSRCostLess(
              C2.ScaleCost, C2.SetupCost);
 }
 
-bool SystemZTTIImpl::areInlineCompatible(const Function *Caller,
-                                         const Function *Callee) const {
-  const TargetMachine &TM = getTLI()->getTargetMachine();
-
-  const FeatureBitset &CallerBits =
-      TM.getSubtargetImpl(*Caller)->getFeatureBits();
-  const FeatureBitset &CalleeBits =
-      TM.getSubtargetImpl(*Callee)->getFeatureBits();
-
-  // Support only equal feature bitsets. Restriction should be relaxed in the
-  // future to allow inlining when callee's bits are subset of the caller's.
-  return CallerBits == CalleeBits;
-}
-
 unsigned SystemZTTIImpl::getNumberOfRegisters(unsigned ClassID) const {
   bool Vector = (ClassID == 1);
   if (!Vector)

llvm/lib/Target/SystemZ/SystemZTargetTransformInfo.h

@@ -65,9 +65,6 @@ class SystemZTTIImpl final : public BasicTTIImplBase<SystemZTTIImpl> {
   bool isLSRCostLess(const TargetTransformInfo::LSRCost &C1,
                      const TargetTransformInfo::LSRCost &C2) const override;
 
-  bool areInlineCompatible(const Function *Caller,
-                           const Function *Callee) const override;
-
   /// @}
 
   /// \name Vector TTI Implementations

llvm/test/Transforms/Inline/SystemZ/inline-target-attr.ll

@@ -12,28 +12,28 @@ entry:
 
 declare i32 @baz(...) #0
 
-define i32 @bar() #1 {
+define i32 @features_subset() #1 {
 entry:
   %call = call i32 @foo()
   ret i32 %call
-; CHECK-LABEL: bar
-; CHECK: call i32 @foo()
+; CHECK-LABEL: features_subset
+; CHECK: call i32 (...) @baz()
 }
 
-define i32 @qux() #0 {
+define i32 @features_equal() #0 {
 entry:
   %call = call i32 @foo()
   ret i32 %call
-; CHECK-LABEL: qux
+; CHECK-LABEL: features_equal
 ; CHECK: call i32 (...) @baz()
 }
 
-define i32 @quux() #2 {
+define i32 @features_different() #2 {
 entry:
-  %call = call i32 @bar()
+  %call = call i32 @foo()
   ret i32 %call
-; CHECK-LABEL: quux
-; CHECK: call i32 @bar()
+; CHECK-LABEL: features_different
+; CHECK: call i32 @foo()
 }