[llvm-bugs] [Bug 44607] New: Relax ARM NEON literal rules

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Tue Jan 21 05:41:24 PST 2020


            Bug ID: 44607
           Summary: Relax ARM NEON literal rules
           Product: clang
           Version: 9.0
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C
          Assignee: unassignedclangbugs at nondot.org
          Reporter: husseydevin at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Currently, the NEON "constant" restrictions are too strict compared to SSE2 and

#include <arm_neon.h>

static inline uint32x4_t shift(uint32x4_t inp, const int amt)
    return vshlq_n_u32(inp, amt);

int main()                                                                    {
    uint32x4_t val = vdupq_n_u32(2384);
    uint32x4_t shifted = shift(val, 3);

`shift` should be constant propagated, and Clang should accept this code.

GCC accepts this code, and Clang also accepts the SSE2 equivalent:

#include <emmintrin.h>

static inline __m128i shift(__m128i val, int amt)
    return _mm_slli_epi32(val, amt);

int main()
    __m128i val = _mm_set1_epi32(2384);
    __m128i shifted = shift(val, 3);

However, I get this with Clang 9.0.1 on Termux aarch64:

neon.cpp:7:12: error: argument to '__builtin_neon_vshlq_n_v' must be a
      constant integer
    return vshlq_n_u32(inp, amt);
           ^                ~~~
      expanded from macro 'vshlq_n_u32'
  __ret = (uint32x4_t) __builtin_neon_vshlq_n_v((int8x16_t)__s0, __p1, 50); \
                       ^                                         ~~~~
1 error generated.

In addition, GCC also converts some things to the non-literal forms. If I
remove the static inline part, I get the following assembly:

        dup     v1.4s, w0
        sshl    v0.4s, v0.4s, v1.4s

This strict literal requirement makes things difficult for things like C++
wrappers, and the requirements should be relaxed like GCC and SSE2.

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