M68k Codegen Bug with -frerun-loop-opt, various EGCS versions

[Melissa O'Neill]

I'm sorry to report a bad code generation bug in EGCS. In egcs-9981012,
this bug manifests itself as accesses to `jp[39 * m]' when the source
said `jp[14 * m]', or accesses to `kp[45 * m]' when the source said 
`kp[9 * m]'  (!!!).

I discovered the bug compiling a fast fourier transform program that's
provided as an example in the Cilk language distribution. I could pinpoint
the problem, but because since I didn't write the code in question myself
(in fact, much of it was machine generated), I wasn't able to make a
short runnable test case. So, I've included two C files, one that can
be run from the command line, and one that just contains an example of
bad code. For the latter, I carefully traced through the code and annotate
the assembler output. I also attempted to determine which part(s) of
the optimizer were to blame.

In all recent m68k versions of egcs I tried (1.1b, 19980921, 19981012),
using `-O2' generated bad code. However, the exact culprit seemed to
vary between versions:

  981012    the bug seems to be tickled caused by the `rerun-loop-opt'
	    optimization, since `-O -frerun-loop-opt' makes bad code
	    and `-O2 -fno-rerun-loop-opt' makes good code

  1.1b      an interaction between several optimizations seems to be
  	    involved, since the following all make good code:
	    	-O2 -fno-strength-reduce
		-O2 -fno-optimize-register-move
		-O2 -fno-regmove
		-O2 -fno-rerun-loop-opt
	    and `-O -fstrength-reduce -foptimize-register-move -fregmove
	    -frerun-loop-opt' makes bad code.

I've enclosed code, both for a runnable program (fft.c -- test with `fft
-n 16384') and a single function from that program where bad code is
generated (fft_twiddle_16.c).  For the latter, I've also included my
annotations on the egcs-9981012 compiler's output for `-O -frerun-loop-opt'
(fft_twiddle_16-O-rlo-annotated.s).  Due to the sizes of these files,
they're enclosed as a compressed tar archive.

    Melissa.

P.S. Even when the code did work, I wasn't very happy about the quality
of the `-O2' optimized code; it's as if peephole optimizer isn't running,
or something.  It was littered with redundant loads, stores and register
moves (stack temporaries were routinely initialized and then almost
immediately overwritten, without their value being read in the meantime).
Curiously, this did not happen with the `-O -frerun-loop-opt' code.

Encs.

Here's the segment of erroneous code, with my annotations. For more
details, look in the tar file.

	lea a5@(a5:l:4),a5	$a5 = m * 5
	movel a5,d3		$d3 = m * 5
	movel d3,d5		$d5 = m * 5
	lsll #4,d5		$d5 = m * 80
	movel d5,a6@(-136)	_tmp34 = m * 80
	movel d1,d5		$d5 = m * 5
	lsll #4,d5		$d5 = m * 80
	movel d5,a6@(-140)	_tmp35 = m * 80
	movel a6@(36),d2	$d2 = m
	lsll #3,d2		$d2 = m * 8
	subl a5,d2		$d2 = m * 3
#######	^----------- bad code? (does the compiler think $a5 = m here too?)
	movel d2,d5		$d5 = m * 3
	lsll #4,d5		$d5 = m * 48
	movel d5,a6@(-144)	_tmp36 = m * 48
	lea a5@(a5:l:8),a5	$a5 = m * 45
#######	^----------- compiler thinks $a5 = m, but $a5 = m * 5		!!!!!!!
	movel a5,d0		$d0 = m * 45
	lsll #3,d0		$d0 = m * 360
	movel d0,a6@(-176)	_tmp44 = m * 360
#######	^----------- bad code, $d0 = m * 360, should be m * 72		!!!!!!!

Here's the tar file with full source:

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