[CUDA] Work around a bug in rint/nearbyint caused by a broken implementation provided...

Normally math functions are forwarded to __nv_* counterparts provided by CUDA's
libdevice bitcode. However, __nv_rint*()/__nv_nearbyint*() functions there have
a bug -- they use round() which rounds *up* instead of rounding towards the
nearest integer, so we end up with rint(2.5f) producing 3.0 instead of expected
2.0. The broken bitcode is not actually used by NVCC itself, which has both a
work-around in CUDA headers and, in recent versions, uses correct
implementations in NVCC's built-ins.

This patch implements equivalent workaround and directs rint*/nearbyint* to
__builtin_* variants that produce correct results.

Differential Revision: https://reviews.llvm.org/D85236

diff --git a/clang/lib/Headers/__clang_cuda_math.h b/clang/lib/Headers/__clang_cuda_math.h
index 332e616..acb26ad 100644 (file)
--- a/clang/lib/Headers/__clang_cuda_math.h
+++ b/clang/lib/Headers/__clang_cuda_math.h
@@ -195,8 +195,8 @@ __DEVICE__ int max(int __a, int __b) { return __nv_max(__a, __b); }
 __DEVICE__ int min(int __a, int __b) { return __nv_min(__a, __b); }
 __DEVICE__ double modf(double __a, double *__b) { return __nv_modf(__a, __b); }
 __DEVICE__ float modff(float __a, float *__b) { return __nv_modff(__a, __b); }
-__DEVICE__ double nearbyint(double __a) { return __nv_nearbyint(__a); }
-__DEVICE__ float nearbyintf(float __a) { return __nv_nearbyintf(__a); }
+__DEVICE__ double nearbyint(double __a) { return __builtin_nearbyint(__a); }
+__DEVICE__ float nearbyintf(float __a) { return __builtin_nearbyintf(__a); }
 __DEVICE__ double nextafter(double __a, double __b) {
   return __nv_nextafter(__a, __b);
 }
@@ -249,8 +249,9 @@ __DEVICE__ double rhypot(double __a, double __b) {
 __DEVICE__ float rhypotf(float __a, float __b) {
   return __nv_rhypotf(__a, __b);
 }
-__DEVICE__ double rint(double __a) { return __nv_rint(__a); }
-__DEVICE__ float rintf(float __a) { return __nv_rintf(__a); }
+// __nv_rint* in libdevice is buggy and produces incorrect results.
+__DEVICE__ double rint(double __a) { return __builtin_rint(__a); }
+__DEVICE__ float rintf(float __a) { return __builtin_rintf(__a); }
 __DEVICE__ double rnorm(int __a, const double *__b) {
   return __nv_rnorm(__a, __b);
 }