Revert "[libc][rpc] Update locking to work on volta"

This reverts commit b132373.

libc/src/__support/CPP/atomic.h

@@ -90,10 +90,6 @@ template <typename T> struct Atomic {
     return __atomic_fetch_or(&val, mask, int(mem_ord));
   }
 
-  T fetch_and(T mask, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
-    return __atomic_fetch_and(&val, mask, int(mem_ord));
-  }
-
   T fetch_sub(T decrement, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
     return __atomic_fetch_sub(&val, decrement, int(mem_ord));
   }

libc/src/__support/GPU/amdgpu/utils.h

@@ -106,7 +106,7 @@ LIBC_INLINE uint32_t get_lane_size() { return LANE_SIZE; }
 
 /// Returns the id of the thread inside of an AMD wavefront executing together.
 [[clang::convergent]] LIBC_INLINE uint32_t get_lane_id() {
-  if constexpr (LANE_SIZE == 64)
+  if (LANE_SIZE == 64)
     return __builtin_amdgcn_mbcnt_hi(~0u, __builtin_amdgcn_mbcnt_lo(~0u, 0u));
   else
     return __builtin_amdgcn_mbcnt_lo(~0u, 0u);
@@ -122,16 +122,6 @@ LIBC_INLINE uint32_t get_lane_size() { return LANE_SIZE; }
   return __builtin_amdgcn_readfirstlane(x);
 }
 
-[[clang::convergent]] LIBC_INLINE uint64_t ballot(uint64_t lane_mask, bool x) {
-  // the lane_mask & gives the nvptx semantics when lane_mask is a subset of
-  // the active threads
-  if constexpr (LANE_SIZE == 64) {
-    return lane_mask & __builtin_amdgcn_ballot_w64(x);
-  } else {
-    return lane_mask & __builtin_amdgcn_ballot_w32(x);
-  }
-}
-
 /// Waits for all the threads in the block to converge and issues a fence.
 [[clang::convergent]] LIBC_INLINE void sync_threads() {
   __builtin_amdgcn_s_barrier();

libc/src/__support/GPU/generic/utils.h

@@ -58,11 +58,6 @@ LIBC_INLINE uint64_t get_lane_mask() { return 1; }
 
 LIBC_INLINE uint32_t broadcast_value(uint32_t x) { return x; }
 
-LIBC_INLINE uint64_t ballot(uint64_t lane_mask, bool x) {
-  (void)lane_mask;
-  return x;
-}
-
 LIBC_INLINE void sync_threads() {}
 
 LIBC_INLINE void sync_lane(uint64_t) {}

libc/src/__support/GPU/nvptx/utils.h

@@ -118,13 +118,6 @@ LIBC_INLINE uint32_t get_lane_size() { return LANE_SIZE; }
 #endif
 }
 
-[[clang::convergent]] LIBC_INLINE uint64_t ballot(uint64_t lane_mask, bool x) {
-#if __CUDA_ARCH__ >= 600
-  return __nvvm_vote_ballot_sync(lane_mask, x);
-#else
-  return lane_mask & __nvvm_vote_ballot(x);
-#endif
-}
 /// Waits for all the threads in the block to converge and issues a fence.
 [[clang::convergent]] LIBC_INLINE void sync_threads() { __syncthreads(); }
 

libc/src/__support/RPC/rpc.h

@@ -107,55 +107,16 @@ template <bool InvertInbox> struct Process {
   }
 
   /// Attempt to claim the lock at index. Return true on lock taken.
-  /// lane_mask is a bitmap of the threads in the warp that would hold the
-  /// single lock on success, e.g. the result of gpu::get_lane_mask()
   /// The lock is held when the zeroth bit of the uint32_t at lock[index]
   /// is set, and available when that bit is clear. Bits [1, 32) are zero.
   /// Or with one is a no-op when the lock is already held.
-  [[clang::convergent]] LIBC_INLINE bool try_lock(uint64_t lane_mask,
-                                                  uint64_t index) {
-    // On amdgpu, test and set to lock[index] and a sync_lane would suffice
-    // On volta, need to handle differences between the threads running and
-    // the threads that were detected in the previous call to get_lane_mask()
-    //
-    // All threads in lane_mask try to claim the lock. At most one can succeed.
-    // There may be threads active which are not in lane mask which must not
-    // succeed in taking the lock, as otherwise it will leak. This is handled
-    // by making threads which are not in lane_mask or with 0, a no-op.
-    uint32_t id = gpu::get_lane_id();
-    bool id_in_lane_mask = lane_mask & (1ul << id);
-
-    // All threads in the warp call fetch_or. Possibly at the same time.
-    bool before =
-        lock[index].fetch_or(id_in_lane_mask, cpp::MemoryOrder::RELAXED);
-    uint64_t packed = gpu::ballot(lane_mask, before);
-
-    // If every bit set in lane_mask is also set in packed, every single thread
-    // in the warp failed to get the lock. Ballot returns unset for threads not
-    // in the lane mask.
-    //
-    // Cases, per thread:
-    // mask==0 -> unspecified before, discarded by ballot -> 0
-    // mask==1 and before==0 (success), set zero by ballot -> 0
-    // mask==1 and before==1 (failure), set one by ballot -> 1
-    //
-    // mask != packed implies at least one of the threads got the lock
-    // atomic semantics of fetch_or mean at most one of the threads for the lock
-    return lane_mask != packed;
+  LIBC_INLINE bool try_lock(uint64_t, uint64_t index) {
+    return lock[index].fetch_or(1, cpp::MemoryOrder::RELAXED) == 0;
   }
 
   // Unlock the lock at index.
-  [[clang::convergent]] LIBC_INLINE void unlock(uint64_t lane_mask,
-                                                uint64_t index) {
-    // Wait for other threads in the warp to finish using the lock
-    gpu::sync_lane(lane_mask);
-
-    // Use exactly one thread to clear the bit at position 0 in lock[index]
-    // Must restrict to a single thread to avoid one thread dropping the lock,
-    // then an unrelated warp claiming the lock, then a second thread in this
-    // warp dropping the lock again.
-    uint32_t and_mask = ~(rpc::is_first_lane(lane_mask) ? 1 : 0);
-    lock[index].fetch_and(and_mask, cpp::MemoryOrder::RELAXED);
+  LIBC_INLINE void unlock(uint64_t, uint64_t index) {
+    lock[index].store(0, cpp::MemoryOrder::RELAXED);
   }
 };
 

libc/src/__support/RPC/rpc_util.h

@@ -9,7 +9,6 @@
 #ifndef LLVM_LIBC_SRC_SUPPORT_RPC_RPC_UTILS_H
 #define LLVM_LIBC_SRC_SUPPORT_RPC_RPC_UTILS_H
 
-#include "src/__support/GPU/utils.h"
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/properties/architectures.h"