[lld] [llvm] Reapply "[RISCV] Support RISCV Atomics ABI attributes (#84597)" (PR #90266)

[Fangrui Song via llvm-commits]

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@@ -1084,10 +1084,62 @@ static void mergeArch(RISCVISAUtils::OrderedExtensionMap &mergedExts,
   }
 }
 
+static void mergeAtomic(DenseMap<unsigned, unsigned>::iterator it,
+                        const InputSectionBase *oldSection,
+                        const InputSectionBase *newSection, unsigned int oldTag,
+                        unsigned int newTag) {
+  using RISCVAttrs::RISCVAtomicAbiTag::AtomicABI;
+  // Same tags stay the same, and UNKNOWN is compatible with anything
+  if (oldTag == newTag || newTag == AtomicABI::UNKNOWN)
+    return;
+
+  switch (oldTag) {
+  case AtomicABI::UNKNOWN:
+    it->getSecond() = newTag;
+    return;
+  case AtomicABI::A6C:
+    switch (newTag) {
+    case AtomicABI::A6S:
+      it->getSecond() = AtomicABI::A6C;
+      return;
+    case AtomicABI::A7:
+      errorOrWarn(toString(oldSection) + " has atomic_abi=" + Twine(oldTag) +
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MaskRay wrote:

We have similar diagnostics in two places. One is OutputSections.cpp:127. Follow its style?

https://github.com/llvm/llvm-project/pull/90266