On Jun 5, 2012, at 1:08 PM, Chandler Carruth <[email protected]> wrote:
> Hey Lang, > > Sorry to jump in late, but was catching on up email and finally read through > this thread. This is the exchange that caught my interest: > > On Fri, Jun 1, 2012 at 4:50 AM, Stephen Canon <[email protected]> wrote: > On May 31, 2012, at 10:40 PM, John McCall <[email protected]> wrote: > > > On May 31, 2012, at 7:22 PM, Lang Hames wrote: > >> Thanks for the suggestion Matthieu. I spoke to Doug and he recommended > >> using attributes rather than a FunctionDecl bit to represent the > >> fp_contract state. > > > > Hmm. I had suggested a bit on FunctionDecl on the assumption that this > > would often be controlled globally, maybe by using a flag to control the > > default or by activating a #pragma before including all the headers. > > Actually, I could even imagine a target (maybe a GPU target?) even > > opting-in to this behavior by default. If we're going to use an Attr, we > > need to make sure it doesn't get added unless the current #pragma state is > > different from the global default; we really don't want to be allocating > > an attribute for every function definition in the translation unit. > > We want FP_CONTRACT ON to be the default for all targets. It's also worth > noting that it's critical that we support setting the pragma to OFF, but in > practice this will be exceedingly rare (almost certainly less than 1% of > sources, and probably far less than that). > > Based on this comment, I'm really not keen on the current representation, but > maybe I've mis-understood it, so I'll ask questions first: > > The 'fmuladd' intrinsic is used to whitelist specific operations for fused > multiply+add handling, correct? Correct. > If so, and if Stephen's stance is correct (I certainly agree with it!) that > this should be allowed for the vast majority of code, that means that almost > every fmul and fadd in the current IR should be a candidate for fusing? Only those that originate from a common source-language *expression*. Your examples should not be fused because the multiply and add are in two separate expressions (which is why we need FE involvement; that information isn't available later). - Steve
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