[PATCH] D16586: Make clang AAPCS compliant w.r.t volatile bitfield accesses

[Richard Smith via cfe-commits]

rsmith added a subscriber: rsmith.

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Comment at: test/CodeGen/aapcs-bitfield.c:312-317
@@ +311,8 @@
+
+  // BE: %[[PTR3:.*]] = bitcast %struct.st5a* %[[PTR2]] to i32*
+  // BE-NEXT: %[[LD:.*]] = load volatile i32, i32* %[[PTR3]], align 4
+  // BE-NEXT: %[[CLR:.*]] = and i32 %[[LD]], -16252929
+  // BE-NEXT: %[[SET:.*]] = or i32 %[[CLR]], 1572864
+  // BE-NEXT: store volatile i32 %[[SET]], i32* %[[PTR3]], align 4
+  m->y.b = 3;
+}
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This violates the C and C++ object models by creating a data race on `m->y.a` that was not present in the source code. A store to a bit-field cannot write to bytes that are not part of the same sequence of bit-field members. If this ABI really requires that (and supports multi-threaded systems), it is not a correct ABI for C11 nor C++11. (This leaves open the question of which standard we should follow...)


http://reviews.llvm.org/D16586