Compiler Bug in 7.4.14

Ralf Goebel

I think I found a compiler bug in CG Tools v7.4.14 when using -O2 on a C6678 CPU. It can be reproduced with the following test code:

void test()
{
	unsigned char Buffer[64];
	int i;

	for (i=0; i<64; i++)
	{
		int pos = i%8;
		if (pos < 3)
			Buffer[3*(i/8)+pos] = i;
	}

	while (1)
		printf("%u\n", Buffer[0]);
}

When calling this function, the for() loop spins forever and the printf() line is never reached. The problem goes away after remove the while(1) line.

Thanks,
Ralf

over 10 years ago

0 George Mock over 10 years ago

TI__Guru**** 251950 points

Please show the exact compiler build options you use.

Thanks and regards,

-George

0 Ralf Goebel over 10 years ago in reply to George Mock

Genius 3715 points

Hi,

these are the compiler options without the include paths:

-mv6600 --abi=eabi -O2 -g --quiet --display_error_number --diag_warning=225 --gen_func_subsections=on --no_bad_aliases --mem_model:data=far --preproc_with_compile --preproc_dependency="test.pp"

Ralf

0 Archaeologist over 10 years ago in reply to Ralf Goebel

TI__Guru* 84285 points

Just to be sure, did you include stdio.h before calling printf?

0 Ralf Goebel over 10 years ago in reply to Archaeologist

Genius 3715 points

Yes, I did include stdio.h. The printf is reached as soon as I remove only the while(1) line.

But the printf() seems not important, I can also put different function call there. Example:

while (1)
memset(Buffer, 0, sizeof(Buffer));

Again, memset() is never reached. The DSP keeps spinning in the for() loop after halting the CPU.

Thanks,
Ralf

0 Archaeologist over 10 years ago in reply to Ralf Goebel

TI__Guru* 84285 points

I can reproduce the problem, but I haven't quite characterized it yet, so I don't know if I can find a workaround. I've submitted SDSCM00052214 to track the issue.

0 pf over 10 years ago in reply to Archaeologist

TI__Expert 4930 points

A workaround is to add "#pragma UNROLL(1)" ahead of the for-loop.

0 pf over 10 years ago

TI__Expert 4930 points

A lovely and bizarre little compiler bug. To make it happen, one needs a loop whose body is a one-armed IF (ie, no "else"), and that that loop be suitable for unrolling and present in a function that ends in an infinite loop. The presence of the infinite loop messes up one of the compiler analyses used in the unrolling.

We've identified a couple of possible solutions and are discussing which one is best. It is likely that one will appear in 7.4.15.

In the meantime, use "#pragma UNROLL(1)" to disable the unrolling, or move the infinite loop out of the function containing the troubled loop; either will avoid the problem.

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Compiler Bug in 7.4.14