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<p class="MsoNormal">Hi everyone,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I noticed today that LLVM’s constant folding of math library functions can lead to minor differences in results. A colleague sent me the following test case which demonstrates the issue:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">#include <stdio.h><o:p></o:p></p>
<p class="MsoNormal">#include <math.h><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">typedef union {<o:p></o:p></p>
<p class="MsoNormal"> double d;<o:p></o:p></p>
<p class="MsoNormal"> unsigned long long i;<o:p></o:p></p>
<p class="MsoNormal">} my_dbl;<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">int main(void) {<o:p></o:p></p>
<p class="MsoNormal"> my_dbl res, x;<o:p></o:p></p>
<p class="MsoNormal"> x.i = 0x3feeb39556255de2ull;<o:p></o:p></p>
<p class="MsoNormal"> res.d = tanh(x.d);<o:p></o:p></p>
<p class="MsoNormal"> printf("tanh(%f) = %f = %016LX\n", x.d, res.d, res.i);<o:p></o:p></p>
<p class="MsoNormal"> return 0;<o:p></o:p></p>
<p class="MsoNormal">}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Compiling with “clang -O2 -g0 -emit-llvm” I get this:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">define dso_local i32 @main() local_unnamed_addr #0 {<o:p></o:p></p>
<p class="MsoNormal"> %1 = tail call double @tanh(double 0x3FEEB39556255DE2) #2<o:p></o:p></p>
<p class="MsoNormal"> %2 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([24 x i8], [24 x i8]* @.str, i64 0, i64 0),<o:p></o:p></p>
<p class="MsoNormal"> double 0x3FEEB39556255DE2, double 0x3FE7CF009CE7F169,<o:p></o:p></p>
<p class="MsoNormal"> i64 4604876745549017449)<o:p></o:p></p>
<p class="MsoNormal"> ret i32 0<o:p></o:p></p>
<p class="MsoNormal">}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We’re still calling ‘tanh’ but all the values passed to printf are constant folded. The constant folding is based on a call to tanh made by the compiler. The problem with this is that if I am linking my program against a different version
of the math library than was used by the compiler I may get a different result.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I can prevent this constant folding with either the ‘nobuiltin’ or ‘strictfp’ attribute. However, it seems to me like this optimization should really be checking the ‘afn’ fast math flag.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Opinions?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Andy<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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