[LLVMbugs] [Bug 11567] New: unnecessary sign extension

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11567

             Bug #: 11567
           Summary: unnecessary sign extension
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: rafael.espindola at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


in
-------------------------------------------
extern void do_something_with(char *arr);

void foo(char **arr, int n) {
   for (T i = 0; i < n; i++) {
     do_something_with(arr[2*i]);
   }
}
------------------------------------------

the loop produced by clang is

movslq %r15d,%r15
mov    (%r14,%r15,8),%rdi
callq  1d <foo+0x1d>
add    $0x2,%r15d
dec    %ebx
jne    11 <foo+0x11>

the loop produced by gcc is

mov    0x0(%rbp),%rdi
inc    %ebx
add    $0x10,%rbp
callq  1d <foo+0x1d>
cmp    %r12d,%ebx
jl     e <foo+0xe>

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