Arm64: In mod operation happening inside the loop, if divisor is an invariant, hoist the divisor checks #64795
Description
Arm64 doesn't have modulo operator, so we convert the operation to div/mul/sub format. However, we don't do much to the tree node until codegen where we generate code for it. During codegen, it is too late to know that divisor was invariant otherwise we can hoist some extra checks like divisor == 0 and divisor == -1.
In below example, when both dividend (x) and divisor (y) are invariant, we hoist the checks properly, but when dividend is not invariant (result) but divisor is invariant (y), we can hoist the checks out of the loop.
public static int issue2(int x, int y, int z)
{
int result = 0;
for (int i = 0; i < z; i++)
{
//result = x % y; <-- this hoist things properly because both dividend and divisor are invariant.
result = result % y;
}
return result;
}...
G_M61875_IG03:
cmp w0, #0 ; Check# 1 divisor == 0, can be hoisted
beq G_M61875_IG08
cmn w0, #1 ; Check# 2 divisor == -1, can be hoisted
bne G_M61875_IG04
adds wzr, w1, w1
bne G_M61875_IG04
bvs G_M61875_IG07
;; bbWeight=4 PerfScore 22.00
G_M61875_IG04:
sdiv w4, w1, w0
mul w4, w4, w0
sub w1, w1, w4
add w3, w3, #1
cmp w3, w2
blt G_M61875_IG03
;; bbWeight=4 PerfScore 62.00
G_M61875_IG05:
mov w0, w1
;; bbWeight=1 PerfScore 0.50
G_M61875_IG06:
ldp fp, lr, [sp],#16
ret lr
;; bbWeight=1 PerfScore 2.00
G_M61875_IG07:
bl CORINFO_HELP_OVERFLOW
;; bbWeight=0 PerfScore 0.00
G_M61875_IG08:
bl CORINFO_HELP_THROWDIVZERO
brk_windows #0
category:cq
theme:div-mod-rem
skill-level:expert
cost:medium
impact:medium