[InstCombine] icmp with sdiv operand transformed strangely

[rotateright]

Bugzilla Link	36240
Version	trunk
OS	All
CC	@majnemer,@RKSimon

Extended Description

define i1 @​cmpdiv(i32 %x) {
%d = sdiv i32 1, %x
%c = icmp slt i32 %d, 0
ret i1 %c
}

$ opt -instcombine cmpsignbit.ll -S
define i1 @​cmpdiv(i32 %x) {
%1 = add i32 %x, 1
%2 = icmp ult i32 %1, 3
%c1 = icmp slt i32 %x, 0
%c = and i1 %2, %c1
ret i1 %c

That's a weird way to write:
%c = icmp eq i32 %x, -1

---

Note that we don't do anything with this case:

define i1 @​cmpdiv400(i32 %x) {
%d = sdiv i32 400, %x
%c = icmp slt i32 %d, 0
ret i1 %c
}

But we could turn this into a range check:

Name: sdiv400
%d = sdiv i32 400, %x
%c = icmp slt i32 %d, 0
=>
%c1 = icmp sge i32 %x, -400
%c2 = icmp slt i32 %x, 0
%c = and i1 %c1, %c2

https://rise4fun.com/Alive/cDM

...so whatever is trying to improve the 1st case could probably use generalizing and fixing.

[majnemer]

It is less strange when you examine:

int f(int x) { return 1 / x; }

You'd get:
define i32 @​f(i32 %x) {
%add = add i32 %x, 1
%cmp = icmp ult i32 %add, 3
%sel = select i1 %cmp, i32 %x, i32 0
ret i32 %sel
}

This is just a cute way of canonicalizing away divisions.