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59deedef1c
Fixes https://bugs.llvm.org/show_bug.cgi?id=35094 The Dead register definition pass should leave alone the atomicrmw instructions on AArch64 (LTE extension). The reason is the following statement in the Arm ARM: "The ST<OP> instructions, and LD<OP> instructions where the destination register is WZR or XZR, are not regarded as doing a read for the purpose of a DMB LD barrier." A good example was given in the gcc thread by Will Deacon (linked in the bugzilla ticket 35094): P0 (atomic_int* y,atomic_int* x) { atomic_store_explicit(x,1,memory_order_relaxed); atomic_thread_fence(memory_order_release); atomic_store_explicit(y,1,memory_order_relaxed); } P1 (atomic_int* y,atomic_int* x) { atomic_fetch_add_explicit(y,1,memory_order_relaxed); // STADD atomic_thread_fence(memory_order_acquire); int r0 = atomic_load_explicit(x,memory_order_relaxed); } P2 (atomic_int* y) { int r1 = atomic_load_explicit(y,memory_order_relaxed); } My understanding is that it is forbidden for r0 == 0 and r1 == 2 after this test has executed. However, if the relaxed add in P1 compiles to STADD and the subsequent acquire fence is compiled as DMB LD, then we don't have any ordering guarantees in P1 and the forbidden result could be observed. Change-Id: I419f9f9df947716932038e1100c18d10a96408d0 llvm-svn: 356360