Xcode Optimizer Bug or stupid mistake? - ios

Question

This code mimics some image processing with malloced memory, it's a distilled example of a problem. It runs fine if optimized at other levels including "Fastest Smallest", but fails on GCC_OPTIMIZATION_LEVEL = 3 AKA Fastest [-03] and Fastest Aggressive. It crashes only on the device, seen on a 6,5s,5 and various IOS 9.3, 8.4.
There's something about the allocation sizes that aggravates the issue. There are some notes in the code about what helps make it fail.
Reproduce by creating an single view app project, set the optimization level to "Fastest" and paste this code into main and call it from inside the autorelease pool, or paste it in the view controller and call it from viewDidLoad or anywhere you like.
The debugger isn't very useful with optimizations turned on, but the crash comes in the while loop at "*writeIter = readIter->d;" a EXC_BAD_ACCESS code=1
So that tells me it's reading and the address that triggers the EXC_BAD_ACCESS is the same as readEnd. That should never happen as that's the condition the while is supposed to prevent... optimizer bug or stupid mistake?
#import <stdlib.h>
#import <stdio.h>
/**
Requires this to fail -> GCC_OPTIMIZATION_LEVEL = 3
This won't do it -> GCC_OPTIMIZATION_LEVEL = s
*/
typedef struct {
unsigned char a, b, c, d;
} foo;
void boom()
{
char* memory[1000];
// these sizes are important to reproducing this issue, changing them by +-1 will make it go away
int height = 960; //480,960,1920
int width = 1280; //640,1280,2560
int depth = sizeof(foo);
printf("height = %d, width = %d, total = %d\n\n", height, width, height*width*depth);
for (int i = 0; i < 1000; ++i)
{
memory[i] = malloc(20000); // allocate memory to force the allocations of readBuf and writeBuf to move, numbers
// less than 15k don't effect the alloced addresses of the bufs, so we keep getting
// the same ones and no boom.
foo* readBuf = malloc(height*width*depth);
unsigned char* writeBuf = malloc(height*width); // smaller than read
foo *readIter = readBuf;
foo *readEnd = readBuf + height*width; // only read size of smaller
unsigned char* writeIter = writeBuf;
printf("test: i = %d, readIter = %p, readEnd = %p, writeIter = %p\n", i, readIter, readEnd, writeIter);
while (readIter < readEnd)
{
*writeIter = readIter->d; // you died here during a read, and readIter == readEnd, look at the EXC_BAD_ACCESS address
// (printfed) it's readEnd, and that isn't supposed to happen with the conditional.
++writeIter;
++readIter;
}
free(readBuf);
free(writeBuf);
}
for (int i = 0; i < 1000; ++i)
{
free(memory[i]);
}
}