[libc] Remove remaining GPU architecture dependent instructions

Summary:
Recent patches have added solutions to the remaining sources of
divergence. This patch simply removes the last occures of things like
`has_builtin`, `ifdef` or builtins with feature requirements. The one
exception here is `nanosleep`, but I made changes in the
`__nvvm_reflect` pass to make usage like this actually work at O0.

Depends on llvm#81331

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

@@ -152,14 +152,7 @@ LIBC_INLINE uint64_t processor_clock() { return __builtin_readcyclecounter(); }
 /// Returns a fixed-frequency timestamp. The actual frequency is dependent on
 /// the card and can only be queried via the driver.
 LIBC_INLINE uint64_t fixed_frequency_clock() {
-  if constexpr (LIBC_HAS_BUILTIN(__builtin_amdgcn_s_sendmsg_rtnl))
-    return __builtin_amdgcn_s_sendmsg_rtnl(0x83);
-  else if constexpr (LIBC_HAS_BUILTIN(__builtin_amdgcn_s_memrealtime))
-    return __builtin_amdgcn_s_memrealtime();
-  else if constexpr (LIBC_HAS_BUILTIN(__builtin_amdgcn_s_memtime))
-    return __builtin_amdgcn_s_memtime();
-  else
-    return 0;
+  return __builtin_readsteadycounter();
 }
 
 /// Terminates execution of the associated wavefront.

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

@@ -138,13 +138,11 @@ LIBC_INLINE uint32_t get_lane_size() { return LANE_SIZE; }
 }
 
 /// Returns the current value of the GPU's processor clock.
-LIBC_INLINE uint64_t processor_clock() {
-  return __nvvm_read_ptx_sreg_clock64();
-}
+LIBC_INLINE uint64_t processor_clock() { return __builtin_readcyclecounter(); }
 
 /// Returns a global fixed-frequency timer at nanosecond frequency.
 LIBC_INLINE uint64_t fixed_frequency_clock() {
-  return __nvvm_read_ptx_sreg_globaltimer();
+  return __builtin_readsteadycounter();
 }
 
 /// Terminates execution of the calling thread.

libc/src/__support/RPC/rpc_util.h

@@ -21,8 +21,9 @@ namespace rpc {
 
 /// Suspend the thread briefly to assist the thread scheduler during busy loops.
 LIBC_INLINE void sleep_briefly() {
-#if defined(LIBC_TARGET_ARCH_IS_NVPTX) && __CUDA_ARCH__ >= 700
-  __nvvm_nanosleep(64);
+#if defined(LIBC_TARGET_ARCH_IS_NVPTX)
+  if (__nvvm_reflect("__CUDA_ARCH") >= 700)
+    LIBC_INLINE_ASM("nanosleep.u32 64;" ::: "memory");
 #elif defined(LIBC_TARGET_ARCH_IS_AMDGPU)
   __builtin_amdgcn_s_sleep(2);
 #elif defined(LIBC_TARGET_ARCH_IS_X86)

libc/src/time/gpu/nanosleep.cpp

@@ -22,14 +22,15 @@ LLVM_LIBC_FUNCTION(int, nanosleep,
   uint64_t nsecs = req->tv_nsec + req->tv_sec * TICKS_PER_NS;
 
   uint64_t start = gpu::fixed_frequency_clock();
-#if defined(LIBC_TARGET_ARCH_IS_NVPTX) && __CUDA_ARCH__ >= 700
+#if defined(LIBC_TARGET_ARCH_IS_NVPTX)
   uint64_t end = start + nsecs / (TICKS_PER_NS / GPU_CLOCKS_PER_SEC);
   uint64_t cur = gpu::fixed_frequency_clock();
   // The NVPTX architecture supports sleeping and guaruntees the actual time
   // slept will be somewhere between zero and twice the requested amount. Here
   // we will sleep again if we undershot the time.
   while (cur < end) {
-    __nvvm_nanosleep(static_cast<uint32_t>(nsecs));
+    if (__nvvm_reflect("__CUDA_ARCH") >= 700)
+      LIBC_INLINE_ASM("nanosleep.u32 %0;" ::"r"(nsecs));
     cur = gpu::fixed_frequency_clock();
     nsecs -= nsecs > cur - start ? cur - start : 0;
   }

libc/src/time/gpu/time_utils.h

@@ -15,24 +15,13 @@ namespace LIBC_NAMESPACE {
 
 #if defined(LIBC_TARGET_ARCH_IS_AMDGPU)
 // AMDGPU does not have a single set frequency. Different architectures and
-// cards can have vary values. Here we default to a few known values, but for
-// complete support the frequency needs to be read from the kernel driver.
-#if defined(__GFX10__) || defined(__GFX11__) || defined(__GFX12__) ||          \
-    defined(__gfx940__) || defined(__gfx941__) || defined(__gfx942__)
-// These architectures use a 100 MHz fixed frequency clock.
-constexpr uint64_t clock_freq = 100000000;
-#elif defined(__GFX9__)
-// These architectures use a 25 MHz fixed frequency clock expect for Vega 10
-// which is actually 27 Mhz. We default to 25 MHz in all cases anyway.
-constexpr uint64_t clock_freq = 25000000;
-#else
-// The frequency for these architecture is unknown. We simply default to zero.
-constexpr uint64_t clock_freq = 0;
-#endif
+// cards can have different values. The actualy frequency needs to be read from
+// the kernel driver and will be between 25 MHz and 100 MHz on most cards. All
+// cards following the GFX9 ISAs use a 100 MHz clock so we will default to that.
+constexpr uint64_t clock_freq = 100000000UL;
 
 // We provide an externally visible symbol such that the runtime can set
-// this to the correct value. If it is not set we try to default to the
-// known values.
+// this to the correct value.
 extern "C" [[gnu::visibility("protected")]] uint64_t
     [[clang::address_space(4)]] __llvm_libc_clock_freq;
 #define GPU_CLOCKS_PER_SEC static_cast<clock_t>(__llvm_libc_clock_freq)