[AIX] Don't crash on unimplemented lowerRelativeReference

[jsji]

Summary

We may call lowerRelativeReference in MC to determine whether target
supports this lowering. We should return nullptr instead of crashing
when we haven't implemented the real lowering.

[hubert.reinterpretcast]

Yes, it looks like the default implementation in the base class just returns nullptr and some other implementations reach points where they return nullptr. LGTM; thanks.

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

2459–2460

Minor nit: formatting.

[jsji]

Format comments.

[nemanjai]

We should add a test case for this. It would be good to verify that the caller does the right thing when we return a nullptr here.

[jsji]

Add reduced testcase.

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

 MCSection *TargetLoweringObjectFileXCOFF::getStaticDtorSection(
   unsigned Priority, const MCSymbol *KeySym) const {
   report_fatal_error("no static destructor section on AIX");
 }
 
 const MCExpr *TargetLoweringObjectFileXCOFF::lowerRelativeReference(
     const GlobalValue *LHS, const GlobalValue *RHS,
     const TargetMachine &TM) const {
-  report_fatal_error("XCOFF not yet implemented.");
+  /* Not implemented yet, but don't crash, return nullptr. */
+  return nullptr;
 }
 
 XCOFF::StorageClass
 TargetLoweringObjectFileXCOFF::getStorageClassForGlobal(const GlobalValue *GV) {
   assert(!isa<GlobalIFunc>(GV) && "GlobalIFunc is not supported on AIX.");
 
   switch (GV->getLinkage()) {
   case GlobalValue::InternalLinkage:

llvm/test/CodeGen/PowerPC/aix-relativeref.ll

+; RUN: llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff  < %s | \
+; RUN:   FileCheck --check-prefix=ASM %s
+
+@__profc_main = private global [1 x i64] zeroinitializer, section "__llvm_prf_cnts", align 8
+@__profd_main = private global { i64, i64, i64, i8*, i8*, i32, [4 x i16] } { i64 -2624081020897602054, i64 742261418966908927, i64 sub (i64 ptrtoint ([1 x i64]* @__profc_main to i64), i64 ptrtoint ({ i64, i64, i64, i8*, i8*, i32, [4 x i16] }* @__profd_main to i64)), i8* bitcast (i32 ()* @main to i8*), i8* null, i32 1, [4 x i16] zeroinitializer }, section "__llvm_prf_data", align 8
+
+; Test fallback of using sub expr for lowerRelativeReference
+define signext i32 @main() {
+; ASM-LABEL: main:
+; ASM:  L..__profd_main:
+; ASM:        .vbyte  8, L..__profc_main-L..__profd_main
+entry:
+  %pgocount = load i64, i64* getelementptr inbounds ([1 x i64], [1 x i64]* @__profc_main, i64 0, i64 0), align 8
+  %0 = add i64 %pgocount, 1
+  store i64 %0, i64* getelementptr inbounds ([1 x i64], [1 x i64]* @__profc_main, i64 0, i64 0), align 8
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval, align 4
+  ret i32 0
+}
+