bug in PEXTRW instruction

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flat assembler > Compiler Internals > bug in PEXTRW instruction

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CandyMan

Joined: 04 Sep 2009
Posts: 294
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop

Code:

pextrw eax,xmm2,1

is assembled as avx512 instruction instead of sse41 (66 0F 3A 15 /r ib PEXTRW m16/r32,xmm,ib)

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17 Jan 2017, 11:22

Tomasz Grysztar
Assembly Artist

Joined: 16 Jun 2003
Posts: 7310
Location: Kraków, Poland

Please provide more context, what version of fasm does this concern, what is the entire source tested, what are the generated bytes, etc.
I have tested this single line with both the fasm 1.71.58 and the latest fasmg macros and they both assemble this instruction as "66 0F C5 C2 01", which is neither SSE4 nor AVX, but plain old SSE2.

17 Jan 2017, 11:48

CandyMan

Joined: 04 Sep 2009
Posts: 294
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop

fasm version 1.71.57

Code:

pextrw [rax],xmm1,1 ;"66 0F 3A 15 08 01" OK
pextrw eax,xmm2,1   ;why not "66 0F 3A 15 D0 01" but "66 0F C5 C2 01" ?

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smaller is better

17 Jan 2017, 12:08

Tomasz Grysztar
Assembly Artist

Joined: 16 Jun 2003
Posts: 7310
Location: Kraków, Poland

CandyMan wrote:

Code:

pextrw eax,xmm2,1   ;why not "66 0F 3A 15 D0 01" but "66 0F C5 C2 01" ?

Because "66 0F C5 C2 01" requires only SSE2 to run and also is shorter.

17 Jan 2017, 12:25

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