[RISCV] Strengthen atomic ordering for sequentially consistent stores

This is a similar change to one proposed for GCC:
https://inbox.sourceware.org/gcc-patches/[email protected]/

The changes in this patch are based on the proposal by Hans Boehm to more
closely match the intended semantics for sequentially consistent stores
and to allow some platforms to avoid an ABI break when switching to more
performant atomic instructions. Platforms that have already compiled
code using the existing mappings will also have more time to gradually
replace that code in preparation of the switch.

Further details can be found in the psABI proposal:
riscv-non-isa/riscv-elf-psabi-doc#378.

This patch implements a mapping that is stronger than the one outlined in table
A.6 of the RISC-V unprivileged spec to be future compatible with table A.7 of
the same document. The related discussion can be found at
https://lists.riscv.org/g/tech-unprivileged/topic/risc_v_memory_model_topics/92916241

The major change to RISC-V code generation is that we will now emit a trailing
fence for sequentially consistent stores.

The new code sequence should have the following form:
```
fence rw,w; s{b|h|w|d}; fence rw,rw;
```

Other changes and optimizations like using amoswap will be handled separately.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D149486

llvm/lib/Target/RISCV/RISCVFeatures.td

@@ -805,6 +805,11 @@ foreach i = {1-31} in
 def FeatureSaveRestore : SubtargetFeature<"save-restore", "EnableSaveRestore",
                                           "true", "Enable save/restore.">;
 
+def FeatureTrailingSeqCstFence : SubtargetFeature<"seq-cst-trailing-fence",
+                                          "EnableSeqCstTrailingFence",
+                                          "true",
+                                          "Enable trailing fence for seq-cst store.">;
+
 def FeatureUnalignedScalarMem
    : SubtargetFeature<"unaligned-scalar-mem", "EnableUnalignedScalarMem",
                       "true", "Has reasonably performant unaligned scalar "

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -16108,6 +16108,9 @@ Instruction *RISCVTargetLowering::emitTrailingFence(IRBuilderBase &Builder,
 
   if (isa<LoadInst>(Inst) && isAcquireOrStronger(Ord))
     return Builder.CreateFence(AtomicOrdering::Acquire);
+  if (Subtarget.enableSeqCstTrailingFence() && isa<StoreInst>(Inst) &&
+      Ord == AtomicOrdering::SequentiallyConsistent)
+    return Builder.CreateFence(AtomicOrdering::SequentiallyConsistent);
   return nullptr;
 }