[LLVMbugs] [Bug 1154] NEW: LICM missing hoist of calls do to missing loop rotation

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Wed Jan 31 20:14:01 PST 2007


           Summary: LICM missing hoist of calls do to missing loop rotation
           Product: libraries
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Loop Optimizer
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sabre at nondot.org

In this example code:

#include <stdlib.h>
#include <string.h>
#define TESTSTRING2 "Dot. date. datum. 123. Some more doubtful demonstration dummy data."
int stringSearch_Clib (long count) {
int i, c = 0;long j;char * p;
char b[sizeof(TESTSTRING2)];

strcpy (b, TESTSTRING2);
for (j=0; j < count; j++) {
for (c=i=0; i < 250; i++) {
if (NULL != (p = strstr (b, "ummy"))) c += (int) (p - b);
if (NULL != (p = strstr (b, " data"))) c += (int) (p - b);
c += (int) strcspn (b, "by");
return c;

The calls to strstr and strcspn are loop invariant and should be hoisted.  They aren't though, because 
the inner loop isn't getting rotated.  This can easily be seen with:

$ llvm-gcc t.c -emit-llvm -O3 -o - -S | llvm-as | opt -print-cfg

and then viewing cfg.stringSearch_Clib.dot.


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