[LLVMbugs] [Bug 8126] New: Strange behaviour with integer promotion on x86-64

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Fri Sep 10 10:06:12 PDT 2010


           Summary: Strange behaviour with integer promotion on x86-64
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: FreeBSD
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: csdavec at swan.ac.uk
                CC: llvmbugs at cs.uiuc.edu

I have some code in a simple inline function:

static INLINE void
GSIMapRightSizeMap(GSIMapTable map, uintptr_t capacity)
  if (3 * capacity >= 4 * map->bucketCount)
      GSIMapResize(map, (3 * capacity)/4 + 1);

This code crashes while evaluating the 4 * map->bucketCount expression with a
segfault.  I've eliminated the possibility that map is an invalid pointer and
added some debugging statements:

printf("Map: %p\n", map);
printf("Map: %lx\n", map->bucketCount);
printf("Map: %lx\n", (uintptr_t)(4 * (int)map->bucketCount));
printf("Map: %lx\n", (4 * map->bucketCount));

Here, map is a pointer to a structure and the bucketCount field is an
uintptr_t, which is a 64-bit quantity on the target architecture (x86-64).  The
output from this is:

Map: 0x802008b10
Map: 0
Map: 0

Program received signal SIGBUS, Bus error.

The first line was just sanity checking, to see if the debugger was getting the
same address for map as the code - it was.  Inspecting this pointer in the
debugger shows that map is a valid value.

The value of the field is 0 - both the debugger and the second print statement
agree on this.

The value of the field cast to an int and multiplied by 4 is 0.

Attempting to multiply the field by a 4, either as the literal 4 or as
((uintptr_t) 4), crashes.

Similarly, this line generates a crash:

uintptr_t p = 4 * map->bucketCount;

However, this works correctly:

uintptr_t p = map->bucketCount * 4;

I therefore assume that something is going a bit wrong with the integer
promotion code.

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