bmiintrin.h: Allow using the tzcnt intrinsics for non-BMI targets

[hans]

Summary

The tzcnt intrinsics are used non non-BMI targets by code (e.g. ffmpeg) that uses it as a potentially faster BSF.

The TZCNT instruction is special in that it's encoded in a backward-compatible way and behaves as BSF on non-BMI targets.

[thakis]

Looks good. The reasoning is that without this, projects will put in an #if __clang__ no tzcnt path (e.g. http://ffmpeg.org/pipermail/ffmpeg-devel/2015-November/183408.html), and with this patch they won't and in a few years when people use build flags that allow BMI instructions, we'll magically start doing the fast thing.

(This is Eric's thing, but he sounded fine with this approach on IRC yesterday.)

[echristo]

Sounds good to me. Weird, but fine :)

-eric

[craig.topper]

The summary of why this is ok is slightly misleading. The backend won't try to encode the TZCNT instruction when the BMI feature is not enabled. Notice this just maps the to the generic non-x86 specific __builtin_ctz. The backend will emit a regular BSF instruction if BMI is not supported. There will be a zero check around it unless the backend can prove that the input is not 0.

cfe/trunk/lib/Headers/bmiintrin.h

 #define _blsi_u32(a)      (__blsi_u32((a)))
 #define _blsmsk_u32(a)    (__blsmsk_u32((a)))
 #define _blsr_u32(a)      (__blsr_u32((a)))
 #define _tzcnt_u32(a)     (__tzcnt_u32((a)))
 
 /* Define the default attributes for the functions in this file. */
 #define __DEFAULT_FN_ATTRS __attribute__((__always_inline__, __nodebug__, __target__("bmi")))
 
-static __inline__ unsigned short __DEFAULT_FN_ATTRS
+/* Allow using the tzcnt intrinsics even for non-BMI targets. Since the TZCNT
+   instruction behaves as BSF on non-BMI targets, there is code that expects
+   to use it as a potentially faster version of BSF. */
+#define __RELAXED_FN_ATTRS __attribute__((__always_inline__, __nodebug__))
+
+static __inline__ unsigned short __RELAXED_FN_ATTRS
 __tzcnt_u16(unsigned short __X)
 {
   return __X ? __builtin_ctzs(__X) : 16;
 }
 
 static __inline__ unsigned int __DEFAULT_FN_ATTRS
 __andn_u32(unsigned int __X, unsigned int __Y)
 {

 }
 
 static __inline__ unsigned int __DEFAULT_FN_ATTRS
 __blsr_u32(unsigned int __X)
 {
   return __X & (__X - 1);
 }
 
-static __inline__ unsigned int __DEFAULT_FN_ATTRS
+static __inline__ unsigned int __RELAXED_FN_ATTRS
 __tzcnt_u32(unsigned int __X)
 {
   return __X ? __builtin_ctz(__X) : 32;
 }
 
 #ifdef __x86_64__
 
 #define _andn_u64(a, b)   (__andn_u64((a), (b)))

 }
 
 static __inline__ unsigned long long __DEFAULT_FN_ATTRS
 __blsr_u64(unsigned long long __X)
 {
   return __X & (__X - 1);
 }
 
-static __inline__ unsigned long long __DEFAULT_FN_ATTRS
+static __inline__ unsigned long long __RELAXED_FN_ATTRS
 __tzcnt_u64(unsigned long long __X)
 {
   return __X ? __builtin_ctzll(__X) : 64;
 }
 
 #endif /* __x86_64__ */
 
 #undef __DEFAULT_FN_ATTRS
+#undef __RELAXED_FN_ATTRS
 
 #endif /* __BMIINTRIN_H */