[llvm-dev] LLVM v3.9.0 and math built-ins

Martin J. O'Riordan via llvm-dev llvm-dev at lists.llvm.org
Tue Sep 20 04:12:35 PDT 2016

Hi Mehdi,


The ISO C specification does permit the math functions to modify ‘errno’, but I thought that the ‘-fno-math-errno’  option was to tell the optimiser to assume that ‘errno’ is not modified by the math functions.  Explicitly providing ‘-fno-math-errno’ is not restoring the elision optimisation that was performed by LLVM v3.8, and this is really only a driver option, with ‘-fmath-errno’ passed onto CC1 if ‘MathErrno’ is true, and omitted if false.


I certainly agree that if ‘MathErrno’ is true, then the compiler should not elide the call as it has to assume that there are memory write side-effects.  It already models the math functions as built-ins and determine the expected answer when the arguments are constants, and elides the tests in the simple example below - all this is good, and when I use ‘-fno-builtin’ it stops correctly disabled this optimisation.


I modified my sources to force ‘readnone’ and even tried ‘setDoesNotAccessMemory()’ on the math functions, but this still results in one call occurring and the code generated is effectively:



return 0;


This is better, but still not as optimal as it was in v3.8.  With this change, compiling with ‘-fmath-errno’ reverts to inserting two calls.


While calling these functions with constants is not terribly common in C code (mostly contrived test cases), it is a lot more common in real C++ code via template expansions and inlining.  With ‘-O3 -S -emit-llvm -fno-math-errno’ plus my change to call ‘setDoesNotAccessMemory()’, the IR generated is as follows:


; ModuleID = 'src/mathElisions.c'

source_filename = "src/mathElisions.c"

target datalayout = "e-m:e-p:32:32-f64:64-i64:64-v128:64-v64:64-v32:32-v16:16-n8:16:32-S64"

target triple = "shave"


; Function Attrs: nounwind readnone

define i32 @useMathName() local_unnamed_addr #0 {

  %1 = tail call float @exp(float 1.000000e+00)

  ret i32 0



; Function Attrs: readnone

declare float @exp(float) local_unnamed_addr #1


; Function Attrs: nounwind

define i32 @useOtherName() local_unnamed_addr #2 {

  %1 = tail call float @foo(float 1.000000e+00) #4

  %2 = fcmp olt float %1, 0x4005AE1480000000

  br i1 %2, label %7, label %3


; <label>:3:                                      ; preds = %0

  %4 = tail call float @foo(float 1.000000e+00) #4

  %5 = fcmp ogt float %4, 0x4005C28F60000000

  br i1 %5, label %7, label %6


; <label>:6:                                      ; preds = %3

  br label %7


; <label>:7:                                      ; preds = %0, %3, %6

  %8 = phi i32 [ 0, %6 ], [ -1, %3 ], [ -1, %0 ]

  ret i32 %8



declare float @foo(float) local_unnamed_addr #3


attributes #0 = { nounwind readnone "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="myriad2.2" "unsafe-fp-math"="false" "use-soft-float"="false" }

attributes #1 = { readnone "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="myriad2.2" "unsafe-fp-math"="false" "use-soft-float"="false" }

attributes #2 = { nounwind "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="myriad2.2" "unsafe-fp-math"="false" "use-soft-float"="false" }

attributes #3 = { "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="myriad2.2" "unsafe-fp-math"="false" "use-soft-float"="false" }

attributes #4 = { nounwind }


Our implementation has C ‘float’ and ‘double’ as 32-bit IEEE Single-Precision and ‘long double’ as 64-bit IEEE Double-Precision.






From: mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>  [mailto:mehdi.amini at apple.com] 
Sent: 16 September 2016 21:16


On Sep 16, 2016, at 1:30 AM, Martin J. O'Riordan via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > wrote:


A little while ago I asked a question on CFE-Dev about a change in the behaviour of programs using the ISO C math functions, although that question should have been put to LLVM-Dev.  But I got excellent clarification of the problem anyway.  However, since then I have been trying to adapt our out-of-tree implementation to get the previous behaviour.  The problem is that something like:


#include <math.h>


extern double foo(double);


int useMathName() {

  if ((exp(1.0) < 2.71) || (exp(1.0) > 2.72))

    return -1;

  return 0;



int useOtherName() {

  if ((foo(1.0) < 2.71) || (foo(1.0) > 2.72))

    return -1;

  return 0;



With v3.8.0 the compiler elided both the calls to ‘exp’ and the tests, so the function ‘useMathName’ reduced to simply:


return 0;


But this was not correct as it ignored the possibility that the math functions could have the side-effect of changing ‘errno’, and this was fixed in v3.9.0, and the calls are no longer elided, though using the “as-if” rule, the tests are still eliminated and ‘useMathName’ becomes:





o me: isn’t the spec saying that errno shall be set on underflow and overflow. The constant folding should be able to detect this and fold if it does not happen?


Also, because we don’t really model errno, we don’t constant fold these call anymore :(


return 0;


So I changed our implementation so that ‘-fno-math-errno’ is the default for the tool-chain, and the ‘-fmath-errno’ option is not passed on to the CC1 phase.  I expected that this would allow the compiler to elide the calls, but it is not doing so.  I don’t want to use ‘-ffast-math’ as this has lots of FP issues, and I don’t want to make it the default.


Any idea why the math functions are not elided?  I am using ‘-O3’ and implying ‘-fno-math-errno’.  I have verified that our Toolchain implements ‘IsMathErrnoDefault’ and returns ‘false’, and that the option ‘-fmath-errno’ is not being passed to the CC1 stage.  Since our math function implementation does not in fact change ‘errno’, it is very desirable that this elision occurs in an FP safe way.


You can check that clang when emitting the IR is adding the attribute "readnone” on the declaration of exp(). If it does not then it assumes errno.


As a starting point, have you tried to pass -fno-math-errno?





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