[clang] [CIR] Add support for discrete bit-field (PR #156085)

[Andy Kaylor via cfe-commits]

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@@ -306,7 +316,45 @@ void CIRRecordLowering::fillOutputFields() {
 RecordDecl::field_iterator
 CIRRecordLowering::accumulateBitFields(RecordDecl::field_iterator field,
                                        RecordDecl::field_iterator fieldEnd) {
-  assert(!cir::MissingFeatures::isDiscreteBitFieldABI());
+  if (isDiscreteBitFieldABI()) {
+    // run stores the first element of the current run of bitfields. fieldEnd is
+    // used as a special value to note that we don't have a current run. A
+    // bitfield run is a contiguous collection of bitfields that can be stored
+    // in the same storage block. Zero-sized bitfields and bitfields that would
+    // cross an alignment boundary break a run and start a new one.
+    RecordDecl::field_iterator run = fieldEnd;
+    // tail is the offset of the first bit off the end of the current run. It's
+    // used to determine if the ASTRecordLayout is treating these two bitfields
+    // as contiguous. StartBitOffset is offset of the beginning of the Run.
+    uint64_t startBitOffset, tail = 0;
+    for (; field != fieldEnd && field->isBitField(); ++field) {
+      // Zero-width bitfields end runs.
+      if (field->isZeroLengthBitField()) {
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andykaylor wrote:

Can you add a test case or two that encounter this condition?

https://github.com/llvm/llvm-project/pull/156085