[PATCH] D55897: Add constrained fptrunc and fpext intrinsics

Fri Jan 25 09:24:45 PST 2019

kpn marked an inline comment as done.
kpn added inline comments.

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Comment at: docs/LangRef.rst:14529
+      @llvm.experimental.constrained.fptrunc(<type> <value>,
+                                          metadata <exception behavior>)
+
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cwabbott wrote:
> Hi,
> 
> I've been working on implementing a Vulkan extension which allows the user to specifiy different rounding modes for the AMDGPU backend. I'm not sure how this works in C/C++, but we're required to support floating-point truncation with non-standard rounding modes. Is there a reason the rounding mode isn't an argument here?
Going back and rereading D43515, I don't see an explicit reason given back then. And I can't find anything in the C99 or IEEE 754 standards, or in the LLVM documentation, that would mandate any particular rounding mode. So I'm open to adding a rounding mode argument to the constrained fptrunc.

Andrew? What do you think of making constrained fptrunc go back to taking a rounding mode argument? 

Having said that, the constrained FP intrinsics are to avoid optimizations that change program behavior taking traps into account. Is this the behavior you need for Vulcan?

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https://reviews.llvm.org/D55897