[fpc-devel] Maybe a bug, maybe not

[J. Gareth Moreton]

Hi everyone,

So I've found a small inconsistency on x86_64 with inline assembly, 
specificially the PINSRW instruction.  Under AT&T notation, you can 
write something like this and it will compile fine with the internal 
assembler:

pinsrw  $0x0, %dx, %xmm0

But if you use -a so the external assembler is used, it throws an error, 
because the second parameter can either be a 16-bit memory location, or 
a 32-bit register... not 16-bit.  In other words, even though the upper 
16 bits aren't used on the register for this instruction, you must 
specify the full 32-bit register:

pinsrw  $0x0, %edx, %xmm0

This is confirmed on page 4-296 of Vol. 2B of the Intel® 64 and IA-32 
Architectures Software Developer’s Manual, showing the second parameter 
is of the form "r32/m16".

Long story short, "pinsrw $0x0, %dx, %xmm0" is technically illegal even 
if it makes more logical sense.  I put this down to PINSRW being a 
carry-over from the MMX instruction set and possibly not designed as 
well as it could be.  Should the internal assembler / syntax checker be 
changed to trap this?

Gareth aka. Kit


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