[Cfp-interest] exceptions and flags

[Ian McIntosh]

In addition to trapping, the PowerPC architecture allows compilers to
detect any exceptions including underflow by using a slightly different
opcode then a conditional branch.  If trapping on exceptions was already
turned on then doing it this way requires turning it off for the duration.

- Ian McIntosh          IBM Canada Lab         Compiler Back End Support
and Development



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  |Jim Thomas <jaswthomas at sbcglobal.net>                                                                                                    |
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  |CFP <cfp-interest at ucbtest.org>,                                                                                                          |
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  |2014-06-04 03:33 PM                                                                                                                      |
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  |Re: [Cfp-interest] exceptions and flags                                                                                                  |
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  |cfp-interest-bounces at oakapple.net                                                                                                        |
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So 754-2008 does not explicitly recommendation a means to detect underflow
exceptions, but its recommendation that languages provide alternate
exception handling entails detecting underflow exceptions. Maybe Part 5
could leave detection of tiny exact underflow cases as a recommended
option, which would allow the simpler slower implementation using flags.

-Jim

On Jun 3, 2014, at 9:32 PM, David Hough CFP <pcfp at oakapple.net> wrote:

>> Does all HW provide a way for SW to detect an underflow exception?
>
> The only way I know is to enable the underflow trap.    If the trap works
> as recommended by 754-1985, then that suffices.
>
> But 754-1985 chapter 8 only recommends traps (to implementations) while
> 754-2008 chapter 8 only recommends alternate exception handling (to
languages).
> Alternate exception handling replaces traps as far as programming
environments
> go, but the traps are still a very likely implementation mechanism.


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