[llvm-dev] Vector trunc code generation difference between llvm-3.9 and 4.0

[Saurabh Verma via llvm-dev]

Correction in the C snippet:

typedef signed short v8i16_t   __attribute__((ext_vector_type(8)));

v8i16_t foo (v8i16_t a, int n)
{
   return a >> n;
}

Best regards
Saurabh



On 17 February 2017 at 16:21, Saurabh Verma <saurabh.verma at movidius.com>
wrote:

> Hello,
>
> We are investigating a difference in code generation for vector splat
> instructions between llvm-3.9 and llvm-4.0, which could lead to a
> performance regression for our target. Here is the C snippet
>
> typedef signed v8i16_t __attribute__((ext_vector_type(8)))
>
> v8i16_t foo (v8i16 a, int n)
> {
>    return result = a >> n;
> }
>
> With llvm-3.9, the generated sequence does a trunc followed by splat, but
> with llvm-4.0 it is reversed to a splat to a bigger vector followed by a
> v8i32->v8i16 trunc. Is this by design? The earlier code sequence is
> definitely better for our target, but are there known scenarios where the
> new sequence would lead to better code?
>
> Here are the instruction sequences generated in the two cases:
>
> With llvm 3.9:
>
> define <8 x i16> @foo(<8 x i16>, i32) #0 {
>   %3 = trunc i32 %1 to i16
>   %4 = insertelement <8 x i16> undef, i16 %3, i32 0
>   %5 = shufflevector <8 x i16> %4, <8 x i16> undef, <8 x i32>
> zeroinitializer
>   %6 = ashr <8 x i16> %0, %5
>   ret <8 x i16> %6
> }
>
>
> With llvm 4.0:
>
> define <8 x i16> @foo(<8 x i16>, i32) #0 {
>   %3 = insertelement <8 x i32> undef, i32 %1, i32 0
>   %4 = shufflevector <8 x i32> %3, <8 x i32> undef, <8 x i32>
> zeroinitializer
>   %5 = trunc <8 x i32> %4 to <8 x i16>
>   %6 = ashr <8 x i16> %0, %5
>   ret <8 x i16> %6
> }
>
> Best regards
> Saurabh Verma
>
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