hi!
I'm using LLVM 3.2. Does it optimize away llvm.pow(llvm.sqrt(x), 2) to `x`
with any settings? I tried with llc -O3, but that didn't do it.
Would be nice to write |v|² in my language ('v' being a 2D vector say
and
|...| and ...² being two separate infix "operators") -- when I could
compare squares of lengths as well as lengths, and know that the sqrt is
optimized away.
Erkki
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That's a pretty seriously unsafe floating point optimization. It could be done in fast-math mode, but I doubt we currently do it. --Owen On Apr 23, 2013, at 1:12 PM, Erkki Lindpere <villane at gmail.com> wrote:> hi! > > I'm using LLVM 3.2. Does it optimize away llvm.pow(llvm.sqrt(x), 2) to `x` with any settings? I tried with llc -O3, but that didn't do it. > > Would be nice to write |v|² in my language ('v' being a 2D vector say and |...| and ...² being two separate infix "operators") -- when I could compare squares of lengths as well as lengths, and know that the sqrt is optimized away. > > Erkki > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
Hi Erkki, On 23/04/13 22:12, Erkki Lindpere wrote:> hi! > > I'm using LLVM 3.2. Does it optimize away llvm.pow(llvm.sqrt(x), 2) to `x` with > any settings? I tried with llc -O3, but that didn't do it.the most powerful optimizations are done by the "opt" tool, not llc. That said, I don't think any pass does this. It would at least require "fast math". Ciao, Duncan.> > Would be nice to write |v|² in my language ('v' being a 2D vector say and |...| > and ...² being two separate infix "operators") -- when I could compare squares > of lengths as well as lengths, and know that the sqrt is optimized away. > > Erkki > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >
Hello, Am Dienstag, 23. April 2013, 13:26:19 schrieb Owen Anderson:> That's a pretty seriously unsafe floating point optimization. It could be > done in fast-math mode, but I doubt we currently do it.I just saw this thread and wonder why it's "seriously" unsafe? I see only one problematic corner case. For x >= 0.0 the result cannot lose precision. For x = NaN the result is the same. sqrt(NaN) = NaN; pow(NaN,2) = NaN For x = +inf the result is the same. sqrt(inf) = inf; pow(inf,2) = inf For negative numbers llvm.sqrt(x) by definition has undefined behavior and so returning x from llvm.pow(llvm.sqrt(x),2) is acceptable. The same should be true for -inf. Only the result for -0.0 changes from 0.0 to -0.0. Is there something I overlooked?> --OwenChristoph> On Apr 23, 2013, at 1:12 PM, Erkki Lindpere <villane at gmail.com> wrote: > > hi! > > > > I'm using LLVM 3.2. Does it optimize away llvm.pow(llvm.sqrt(x), 2) to `x` > > with any settings? I tried with llc -O3, but that didn't do it. > > > > Would be nice to write |v|² in my language ('v' being a 2D vector say and > > |...| and ...² being two separate infix "operators") -- when I could > > compare squares of lengths as well as lengths, and know that the sqrt is > > optimized away. > > > > Erkki > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev