[llvm-commits] [llvm-gcc-4.2] r82640 - /llvm-gcc-4.2/trunk/gcc/llvm-convert.cpp

Chris Lattner clattner at apple.com
Sat Sep 26 01:33:48 CDT 2009

On Sep 25, 2009, at 2:02 PM, Dale Johannesen wrote:

> I haven't tried to touch clang.  I think the rest of it is done.

Great, thanks Dale!

> (Should probably mention sqrt, and other math intrinsics, are  
> readonly not readnone because of:
> // These functions do not actually read memory, but they are  
> sensitive to the
> // rounding mode.  This needs to be modelled separately; in the  
> meantime
> // declaring them as reading memory is conservatively correct.)

Wow, that's a gross hack.  Hopefully someday we will have a chance to  
do the right thing.  Thanks again for untangling this snarl,


> On Sep 23, 2009, at 11:04 PM, Chris Lattner wrote:
>> On Sep 23, 2009, at 6:10 PM, Evan Cheng wrote:
>> On Sep 23, 2009, at 3:21 PM, Dale Johannesen wrote:
>>>> On Sep 23, 2009, at 3:18 PMPDT, Evan Cheng wrote:
>>>>> Are we generating a call to sqrt now? If so, that's bad. We should
>>>>> be using SSE sqrts* instructions.
>>>> Agreed.  My proposed semantic change would fix that.
>>> You mean change to llvm.sqrt and then llvm-gcc can switch bad to
>>> generating the intrinsic? It seems like the current fix is not  
>>> what we
>>> want. Perhaps we should revert it first?
>>> According to Chris, the semantics of sqrt of negative value is  
>>> defined
>>> and this is just some optimization bug.
>> Sorry, I'm just catching up on this now.  There are a couple of  
>> things that confuse the issue, but I'll just try to keep it "to the  
>> point" instead of rambling about history.
>> 1. I did tell Evan that sqrt is defined on negative number, but I  
>> misunderstood and didn't think about llvm.sqrt.  Please disregard  
>> my comment Evan.  llvm.sqrt should be undefined on negative numbers  
>> as langref says, and llvm-gcc/clang should only transform sqrt to  
>> llvm.sqrt if the appropriate "I don't care about fp semantics" flag  
>> is set.
>> 2. Dale's patch to llvm-gcc is ok, but it would be better to still  
>> do the transformation when -ffast-math is specified or whatever the  
>> more precise "nan's aren't generated" flag is.  We should do the  
>> same thing for clang as well.
>> 3. Please make sure that llvm-gcc/clang on the mac (and other  
>> targets with -fno-math-errno) are producing a call to sqrt that is  
>> marked as readnone.  Given this, the mid-level optimizer should  
>> hoist and cse the calls to sqrt just as well as it did calls to  
>> llvm.sqrt.
>> 4. The constant folding of llvm.sqrt(-123) -> 0 is ok because the  
>> intrinsic really is undefined on negative.  The constant folding of  
>> sqrt(-123) doesn't fold if the input is negative, so it will just  
>> not optimize the curious case.
>> 5. Please make the X86 backend compile calls to readonly/readnone  
>> "sqrt" produce a sqrtsd (etc) instruction.  We really don't want to  
>> get a function call on the mac (or other x86 target with -fno-math- 
>> errno).  Like malloc, if someone cares about -fno-builtin-sqrt,  
>> they can solve the general problem.  We already constant fold "real  
>> sqrt" calls in Analysis/ConstantFolding.cpp anyway.
>> 6. Please add handling of "real sqrt" to llvm::CannotBeNegativeZero  
>> to match the handling of llvm.sqrt.
>> Thanks!
>> -Chris

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