[9fans] 5l bug

[Paul Patience <paul-a.patience@polymtl.ca>]

Mischief reported a crash with on arm with
winwatch when closing all windows excluding those
ignored through the -e flag. Cinap_lenrek
narrowed the problem down to a bug in the 5l
linker: it generates incorrect code when using
the conditional operator and dividing.
A minimal test case is the following:

term% cat foo.c
#include <u.h>
#include <libc.h>

void
main(void)
{
	int i, j, k;

	i = 1;
	j = 0;
	k = j <= 0 ? 1 : 2/i;
	print("%d\n", k);
	exits(nil);
}

The output of asm(main) in acid is:

acid: asm(main)
main 0x00001020	MOVW.W	R14,#-0x18(R13)
main+0x4 0x00001024	MOVW	$#0x1,R1
main+0x8 0x00001028	MOVW	$#0x0,R2
main+0xc 0x0000102c	CMP.S	$#0x0,R2
main+0x10 0x00001030	MOVW.LE	R1,R3
main+0x14 0x00001034	MOVW.GT	$#0x2,R3
main+0x18 0x00001038	SUB.GT	$#0x8,R13,R13
main+0x1c 0x0000103c	MOVW	R1,#0x4(R13)
main+0x20 0x00001040	MOVW	R3,R11
main+0x24 0x00001044	BL	_div
main+0x28 0x00001048	MOVW	R11,R3
main+0x2c 0x0000104c	ADD	$#0x8,R13,R13
main+0x30 0x00001050	MOVW	$#0x7080,R0
main+0x34 0x00001054	MOVW	R3,#0x8(R13)
main+0x38 0x00001058	BL	print
main+0x3c 0x0000105c	MOVW	$#0x0,R0
main+0x40 0x00001060	BL	exits
main+0x44 0x00001064	RET.P	#0x18(R13)

The problem is that the division at main+0x24
happens regardless of the comparison. The output
from the compiler is correct, however:

term% 5c -S foo.c
	TEXT	main+0(SB),0,$20
	MOVW	$1,R1
	MOVW	$0,R2
	CMP	$0,R2,
	MOVW.LE	R1,R3
	MOVW.GT	$2,R3
	DIV.GT	R1,R3,R3
	MOVW	$.string<>+0(SB),R0
	MOVW	R3,8(R13)
	BL	,print+0(SB)
	MOVW	$0,R0
	BL	,exits+0(SB)
	RET	,
	DATA	.string<>+0(SB)/8,$"%d\n\z\z\z\z\z"
	GLOBL	.string<>+0(SB),$8
	END	,

A (temporary) workaround for this is using an
if-else statement, for which the linker generates
correct code.

--
pap