[LLVMbugs] [Bug 8100] New: No way to prevent excessive constant propagation

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=8100

           Summary: No way to prevent excessive constant propagation
           Product: new-bugs
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: marc.glisse at normalesup.org
                CC: llvmbugs at cs.uiuc.edu


Hello,

consider this code:

#include <fenv.h>
#include <assert.h>

int main(){
        double a=1.1;
        double b=10.1;
        fesetround(FE_UPWARD);
        assert(-((-a)*b)!=a*b);
}

Excessive optimization will remove the double negation. gcc has option
-frounding-math to help with this. Intel's -fp-model strict is not quite as
good (it fails for this example) but it helps (for instance it lets the code
work if I write 1.1 and 10.1 instead of a and b in the assertion). With
llvm-gcc or clang, this always fails at -O1.

A similar option for llvm would be very welcome, there are applications where
rounding in the appropriate direction is crucial (precision and speed come far
behind).

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