Enable this in CS for PowerPC; disable it for all other machines.
PowerPC has no remainder instruction; the back end uses division to
compute remainder. If CS finds both a / b and a % b, then CS now
rewrites a % b as a - b * (a / b) and computes a / b only once. This
removes an extra division in the PowerPC code, so it saves both time
and space.
I have not considered whether to enable this optimization for other
machines. It might be less useful in machines with a remainder
instruction. Also, if a % b occurs before a / b, the EM code gets a
DUP. PowerPC ncg handles this DUP well; other back ends might not.
In ego, the CS phase may convert a LAR/SAR to AAR LOI/STI so it can
optimize multiple occurrences of AAR of the same array element. This
conversion should not happen if it would LOI/STI a large or unknown
size.
cs_profit.c okay_lines() checked the size of each occurrence of AAR
except the first. If the first AAR was the implicit AAR in a LAR/SAR,
then the conversion happened without checking the size. For unknown
size, this made a bad LOI -1 or STI -1. Fix by checking the size
earlier: if a LAR/SAR has a bad size, then don't enter it as an AAR.
This Modula-2 code showed the bug. Given M.def:
DEFINITION MODULE M;
TYPE S = SET OF [0..95];
PROCEDURE F(a: ARRAY OF S; i, j: INTEGER);
END M.
and M.mod:
(*$R-*) IMPLEMENTATION MODULE M;
FROM SYSTEM IMPORT ADDRESS, ADR;
PROCEDURE G(s: S; p, q: ADDRESS; t: S); BEGIN
s := s; p := p; q := q; t := t;
END G;
PROCEDURE F(a: ARRAY OF S; i, j: INTEGER); BEGIN
G(a[i + j], ADR(a[i + j]), ADR(a[i + j]), a[i + j])
END F;
END M.
then the bug caused an error:
$ ack -mlinuxppc -O3 -c.e M.mod
/tmp/Ack_b357d.g, line 57: Argument range error
The bug had put LOI -1 in the code, then em_decode got an error
because -1 is out of range for LOI.
Procedure F has 4 occurrences of `a[i + j]`. The size of `a[i + j]`
is 96 bits, or 12 bytes, but the EM code hides the size in an array
descriptor, so the size is unknown to CS. The pragma `(*$R-*)`
disables a range check on `i + j` so CS can work. EM uses AAR for the
2 `ADR(a[i + j])` and LAR for the other 2 `a[i + j]`. EM pushes the
arguments to G in reverse order, so the last `a[i + j]` in Modula-2 is
the first LAR in EM.
CS found 4 occurrences of AAR. The first AAR was an implicit AAR in
LAR. Because of the bug, CS converted this LAR 4 to AAR 4 LOI -1.
