[LLVMbugs] [Bug 11376] New: illegal load widening at O2

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Mon Nov 14 15:44:32 PST 2011


             Bug #: 11376
           Summary: illegal load widening at O2
           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: kcc at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Three one-byte loads are illegally combined into one 4-byte load 
Observed on r144590
The test below triggers SEGV, but the bug is obvious from disassembly. 

% cat t.cc 
#include <sys/mman.h>
#include <stdio.h>
#include <assert.h>
#include <stdlib.h>

inline char Check(const char& v) {
  return v ? 1 : 0;

char foo(char *X) {
  return Check(X[0]) | Check(X[1]) | Check(X[2]);

static const int kPageSize = 4096;

int main() {
  // poor man's electric fence:
  // allocate two pages and lock the second one.
  char *mem = (char*)mmap(0, kPageSize * 2, PROT_READ | PROT_WRITE,
                          MAP_PRIVATE|MAP_ANON, 0, 0);
  assert(mem != (char*)-1);
  mprotect(mem + kPageSize, kPageSize, PROT_NONE);
  fprintf(stderr, "mem = %p\n", mem);
  char *X = mem + kPageSize - 3;
  fprintf(stderr, "X = %p\n", X);
  // We can access X[0], X[1], X[2], but not X[3].
  char res = foo(X);
  fprintf(stderr, "PASSED %d\n", res);

% clang++ t.cc -O2 ; ./a.out 
mem = 0x7f1313ca0000
X = 0x7f1313ca0ffd
Segmentation fault

00000000004006b0 <_Z3fooPc>:
  4006b0:       8b 07                   mov    (%rdi),%eax   <<<<<<<<<<<<<<<
  4006b2:       89 c1                   mov    %eax,%ecx
  4006b4:       c1 e9 08                shr    $0x8,%ecx
  4006b7:       09 c1                   or     %eax,%ecx
  4006b9:       c1 e8 10                shr    $0x10,%eax
  4006bc:       08 c8                   or     %cl,%al
  4006be:       0f 95 c0                setne  %al
  4006c1:       0f b6 c0                movzbl %al,%eax
  4006c4:       c3                      retq   
  4006c5:       66 66 2e 0f 1f 84 00    nopw   %cs:0x0(%rax,%rax,1)
  4006cc:       00 00 00 00 

The same test passes at O1.

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