[LLVMbugs] [Bug 1253] NEW: "optimized" range comparison not simplified

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=1253

           Summary: "optimized" range comparison not simplified
           Product: new-bugs
           Version: unspecified
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: baldrick at free.fr


The condition a<=x<=b can be transformed into (unsigned)n-a<=b-a.
However if a=INT_MIN then this "optimization" leads to suboptimal
code:

#include <limits.h>
int bad (int n) {
  return ((unsigned)n-INT_MIN<=-1-INT_MIN);
}

->

define i32 @bad(i32 %n) {
entry:
        %tmp2 = xor i32 %n, -2147483648         ; <i32> [#uses=1]
        icmp sgt i32 %tmp2, -1          ; <i1>:0 [#uses=1]
        zext i1 %0 to i32               ; <i32>:0 [#uses=1]
        ret i32 %0
}

This would be better as: "return n<0;"



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