[LLVMdev] optimization in presence of floating point

[Hal Finkel]

----- Original Message -----
> From: "reed kotler" <rkotler at mips.com>
> To: LLVMdev at cs.uiuc.edu
> Sent: Wednesday, April 17, 2013 8:32:23 PM
> Subject: [LLVMdev] optimization in presence of floating point
> 
> an interesting problem occurs if you do interval arithmetic.
> 
> void foo() {
> float XL, XU, Y, Z;
> 
> ....
> call_change_rounding_mode(lower);
> XL = Y + Z; // lower bound
> call_change_rounding_mode(upper);
> XU = Y + Z;
> 
> }
> 
> Two issues here:
> 1) will the compiler mistakenly treat Y+Z as a common subexpression.
> 2) might it move the call_change_rounding_mode after the assignment
> to
> XU since it seens no dependency.

I have come across these issues myself as well. To be fair, Clang does not support the FENV_ACCESS pragma, and as I recall, the standard allows an implementation-defined default value (which, in our case, cannot be changed). Moreover, LLVM internally does not have an option to enable the correct conservative side-effect model to make all of this really work correctly.

 -Hal

> 
> This is actually to my previous mips16 ir post but is a question of
> more
> wide interest.
> 
> 
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