[fpc-devel] Successful implementation of inlinesupportforpureassembler routines on x86

[Jonas Maebe]

On 18/03/2019 11:36, J. Gareth Moreton wrote:
> I think that is the fundamental difference between intrinsics and 
> reusable, inlinable assembly language... you would use intrinsics mostly 
> to use one or two special instructions in a convenient, concise way, 
> while assembly language is a "give me all the power!" request. 
> Admittedly though my patch does allow the peephole optimizer to touch 
> the assembly language in the inlined procedure, mostly to help optimize 
> parameter and result passing.

No optimizer must ever be allowed to interact with inline assembly 
(apart from parametrised operands related to constraints). Inline 
assembly is completely the responsibility of the programmer, and must 
always appear literally in the output. It is impossible to analyse all 
possible tricks a programmer may use and safely handle those in an 
optimizer.

Similarly, replacing return instructions with jumps is something you 
cannot do. You can do that with Pascal-level "exit" statements, but not 
with assembler-level return instructions. As you wrote yourself: "If the 
program breaks because of what they've coded, it's on them to fix it."

That is again why I argue for never allowing the inlining of pure 
assembler routines, and only for inlining Pascal routines that happen to 
also contain inline assembly. In that case, you will (almost?) never 
encounter the case where a routine works if it's not inlined, but breaks 
once it's inlined, without needing any warnings or target-specific analyses.


Jonas