[LLVMbugs] [Bug 5768] New: Invalid codegen for shifts

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Sat Dec 12 10:21:59 PST 2009


           Summary: Invalid codegen for shifts
           Product: clang
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: asl at math.spbu.ru
                CC: llvmbugs at cs.uiuc.edu

Consider the following code:

#include <stdint.h>

uint8_t lshr8(uint8_t a, uint8_t cnt) {
  return a >> cnt;

"clang -ccc-host-triple msp430-elf foo.c -O3 -emit-llvm" yields

define zeroext i8 @lshr8(i8 zeroext %a, i8 zeroext %cnt) nounwind readnone {
  %conv = zext i8 %a to i16
  %conv2 = zext i8 %cnt to i16
  %shr = lshr i16 %conv, %conv2
  %conv3 = trunc i16 %shr to i8
  ret i8 %conv3

note that useless ext's / truncs were added. They are not removed by optimizers
(this is another problem)

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