[cfe-dev] ordering of asm constraints in clang CodeGen

[Chris Lattner]

On Oct 12, 2009, at 4:54 AM, Roman Divacky wrote:

> hi
>
> this simple code (coming from bug 879) on X86:
>
> double
> ldexp(double value, int exp)
> {
>        double temp, texp, temp2;
>        texp = exp;
>
>        __asm ("fscale " : "=u" (temp2), "=t" (temp) : "0" (texp), "1"
> (value));
>        return (temp);
> }
>
> is being handled by CGStmt.cpp:EmitAsmStmt(), the construction of  
> output
> constraints (ie. "=u" and "=t") is done simply from left to right.  
> so this
> particular example will construct output constraints of:

Hi Roman,

As we discussed on IRC, this is really an llvm backend issue.  My  
memory is fuzzy here, but last time I tried to tackle this, I ran into  
problems where we really wanted a single merge FpGET_ST0/1 instruction  
produced by isel (I think) that the fpstackifier could deal with as  
one operation.  If we don't have that, then other load/stores etc can  
get in between the machine instructions.

-Chris

>
>
> 			st(1), st
>
>
> according to X86FloatingPoint.cpp:handleSpecialFP()
>
>  // FpGET_ST1 should occur right after a FpGET_ST0 for a call or  
> inline asm.
>  // The pattern we expect is:
>  //  CALL
>  //  FP1 = FpGET_ST0
>  //  FP4 = FpGET_ST1
>  //
>  // At this point, we've pushed FP1 on the top of stack, so it  
> should be
>  // present if it isn't dead.  If it was dead, we already emitted a  
> pop to
>  // remove it from the stack and StackTop = 0.
>
> (note that st(0) is the same as st)
> the expected order of constrains is FpGET_ST0 and then FpGET_ST1  
> while clang
> gives it in reverse. This leads to this assertion being hit:
>
> Assertion failed: (StackTop == 0 && "Stack should be empty after a  
> call!"), function handleSpecialFP, file
> X86FloatingPoint.cpp, line 964.
>
> can someone shed some light on this? is it true that llvm expects  
> this order?
> if so shouldnt clang order it this way? I need a little info about  
> the situation
> to be able to fix it.
>
> Thank you!
>
> Roman Divacky
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