[AtomicExpand] Avoid creating an unnamed libcall

[arichardson]

Summary

I recently modified this pass to better support CHERI-RISC-V and while
doing so I noticed that this pass was calling M->getOrInsertFunction()
with the result of TLI->getLibcallName(RTLibType). However, AMDGPU fills
the libcalls array with nullptr, so this creates an anonymous function
instead. This patch changes expandAtomicOpToLibcall to return false in
case the libcall does not exist and changes the assert() in the callees to
a report_fatal_error instead.

[arsenm]

Arguably this could be a proper context error but this is an improvement

llvm/lib/CodeGen/AtomicExpandPass.cpp

void AtomicExpand::expandAtomicLoadToLibcall(LoadInst *I) {
   static const RTLIB::Libcall Libcalls[6] = {
       RTLIB::ATOMIC_LOAD,   RTLIB::ATOMIC_LOAD_1, RTLIB::ATOMIC_LOAD_2,
       RTLIB::ATOMIC_LOAD_4, RTLIB::ATOMIC_LOAD_8, RTLIB::ATOMIC_LOAD_16};
   unsigned Size = getAtomicOpSize(I);
 
   bool expanded = expandAtomicOpToLibcall(
       I, Size, I->getAlign(), I->getPointerOperand(), nullptr, nullptr,
       I->getOrdering(), AtomicOrdering::NotAtomic, Libcalls);
-  (void)expanded;
-  assert(expanded && "expandAtomicOpToLibcall shouldn't fail tor Load");
+  if (!expanded)
+    report_fatal_error("expandAtomicOpToLibcall shouldn't fail for Load");
 }
 
 void AtomicExpand::expandAtomicStoreToLibcall(StoreInst *I) {
   static const RTLIB::Libcall Libcalls[6] = {
       RTLIB::ATOMIC_STORE,   RTLIB::ATOMIC_STORE_1, RTLIB::ATOMIC_STORE_2,
       RTLIB::ATOMIC_STORE_4, RTLIB::ATOMIC_STORE_8, RTLIB::ATOMIC_STORE_16};
   unsigned Size = getAtomicOpSize(I);
 
   bool expanded = expandAtomicOpToLibcall(
       I, Size, I->getAlign(), I->getPointerOperand(), I->getValueOperand(),
       nullptr, I->getOrdering(), AtomicOrdering::NotAtomic, Libcalls);
-  (void)expanded;
-  assert(expanded && "expandAtomicOpToLibcall shouldn't fail tor Store");
+  if (!expanded)
+    report_fatal_error("expandAtomicOpToLibcall shouldn't fail for Store");
 }
 
 void AtomicExpand::expandAtomicCASToLibcall(AtomicCmpXchgInst *I) {
   static const RTLIB::Libcall Libcalls[6] = {
       RTLIB::ATOMIC_COMPARE_EXCHANGE,   RTLIB::ATOMIC_COMPARE_EXCHANGE_1,
       RTLIB::ATOMIC_COMPARE_EXCHANGE_2, RTLIB::ATOMIC_COMPARE_EXCHANGE_4,
       RTLIB::ATOMIC_COMPARE_EXCHANGE_8, RTLIB::ATOMIC_COMPARE_EXCHANGE_16};
   unsigned Size = getAtomicOpSize(I);
 
   bool expanded = expandAtomicOpToLibcall(
       I, Size, I->getAlign(), I->getPointerOperand(), I->getNewValOperand(),
       I->getCompareOperand(), I->getSuccessOrdering(), I->getFailureOrdering(),
       Libcalls);
-  (void)expanded;
-  assert(expanded && "expandAtomicOpToLibcall shouldn't fail tor CAS");
+  if (!expanded)
+    report_fatal_error("expandAtomicOpToLibcall shouldn't fail for CAS");
 }
 
 static ArrayRef<RTLIB::Libcall> GetRMWLibcall(AtomicRMWInst::BinOp Op) {
   static const RTLIB::Libcall LibcallsXchg[6] = {
       RTLIB::ATOMIC_EXCHANGE,   RTLIB::ATOMIC_EXCHANGE_1,
       RTLIB::ATOMIC_EXCHANGE_2, RTLIB::ATOMIC_EXCHANGE_4,
       RTLIB::ATOMIC_EXCHANGE_8, RTLIB::ATOMIC_EXCHANGE_16};
   static const RTLIB::Libcall LibcallsAdd[6] = {
bool AtomicExpand::expandAtomicOpToLibcall(
   } else if (Libcalls[0] != RTLIB::UNKNOWN_LIBCALL) {
     RTLibType = Libcalls[0];
   } else {
     // Can't use sized function, and there's no generic for this
     // operation, so give up.
     return false;
   }
 
+  if (!TLI->getLibcallName(RTLibType)) {
+    // This target does not implement the requested atomic libcall so give up.
+    return false;
+  }
+
   // Build up the function call. There's two kinds. First, the sized
   // variants.  These calls are going to be one of the following (with
   // N=1,2,4,8,16):
   //  iN    __atomic_load_N(iN *ptr, int ordering)
   //  void  __atomic_store_N(iN *ptr, iN val, int ordering)
   //  iN    __atomic_{exchange|fetch_*}_N(iN *ptr, iN val, int ordering)
   //  bool  __atomic_compare_exchange_N(iN *ptr, iN *expected, iN desired,
   //                                    int success_order, int failure_order)

llvm/test/Transforms/AtomicExpand/AMDGPU/unaligned-atomic.ll

-; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -atomic-expand %s | FileCheck -check-prefix=GCN %s
-
-; FIXME: This should not introduce a libcall, much less one to an
-; anonymous function.
+; RUN: not --crash opt -S -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -atomic-expand %s 2>&1 | FileCheck %s
+; The AtomicExpand pass cannot handle missing libcalls (yet) so reports a fatal error.
+; CHECK: LLVM ERROR: expandAtomicOpToLibcall shouldn't fail for Load
 
 define i32 @atomic_load_global_align1(i32 addrspace(1)* %ptr) {
 ; GCN-LABEL: @atomic_load_global_align1(
 ; GCN-NEXT:    [[TMP1:%.*]] = bitcast i32 addrspace(1)* [[PTR:%.*]] to i8 addrspace(1)*
 ; GCN-NEXT:    [[TMP2:%.*]] = addrspacecast i8 addrspace(1)* [[TMP1]] to i8*
 ; GCN-NEXT:    [[TMP3:%.*]] = alloca i32, align 4
 ; GCN-NEXT:    [[TMP4:%.*]] = bitcast i32* [[TMP3]] to i8*
 ; GCN-NEXT:    call void @llvm.lifetime.start.p0i8(i64 4, i8* [[TMP4]])