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From: Joseph Myers <>
To: Aldy Hernandez <>
Cc: Jakub Jelinek <>,
	GCC patches <>,
	Andrew MacLeod <>
Subject: Re: [PATCH] [range-ops] Implement sqrt.
Date: Thu, 17 Nov 2022 18:59:45 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, 17 Nov 2022, Aldy Hernandez via Gcc-patches wrote:

> So... is the optimization wrong?  Are we not allowed to substitute
> that NAN if we know it's gonna happen?  Should we also allow F F F F F
> in the test?  Or something else?

This seems like the usual ambiguity about what transformations 
-ftrapping-math (on by default) is meant to prevent.

Generally it's understood to prevent transformations that add *or remove* 
exceptions, so folding a case that raises "invalid" to a NaN (with 
"invalid" no longer raised) is invalid with -ftrapping-math.  But that 
doesn't tend to be applied if the operation raising the exceptions has a 
result that is otherwise unused - in such a case the operation may still 
be removed completely (the exception isn't properly treated as a side 
effect to avoid dead code elimination; cf. Marc Glisse's -ffenv-access 
patches from August 2020).  And it may often also not be applied to 

There have been various past discussions of possible ways to split up the 
different effects of options such as -ftrapping-math into finer-grained 
options allowing more control of what transformations are permitted - see 
and bug 54192.  There is also the question in that context of which 
sub-options should be enabled by default at all.

Joseph S. Myers

  parent reply	other threads:[~2022-11-17 18:59 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-13 20:05 Aldy Hernandez
2022-11-13 20:39 ` Jakub Jelinek
2022-11-14  7:45   ` Aldy Hernandez
2022-11-14 14:30     ` Jeff Law
2022-11-14 14:35       ` Jakub Jelinek
2022-11-14 14:48         ` Jeff Law
2022-11-14 15:01         ` Aldy Hernandez
2022-11-14 21:55   ` Joseph Myers
2022-11-16 20:32     ` Jakub Jelinek
2022-11-17 16:40       ` Aldy Hernandez
2022-11-17 16:48         ` Aldy Hernandez
2022-11-17 17:42           ` Aldy Hernandez
2022-11-17 18:59         ` Joseph Myers [this message]
2022-11-17 19:37           ` Jakub Jelinek
2022-11-17 20:43             ` Joseph Myers
2022-11-18  8:39             ` Richard Biener
2022-11-18 10:37               ` Aldy Hernandez
2022-11-18 10:44                 ` Jakub Jelinek
2022-11-18 11:20                   ` Aldy Hernandez
2022-11-18 11:57                     ` Aldy Hernandez
2022-11-18 12:14                   ` Richard Biener

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