Invalid division instcombine

[efriedma-quic]

Bugzilla Link	1931
Resolution	INVALID
Resolved on	Jan 27, 2008 11:28
Version	unspecified
OS	Linux

Extended Description

define i32 @​fold(i32 %a) {
%b = sdiv i32 %a, -1
%c = sdiv i32 %b, -2
ret i32 %c
}

gets combined into

define i32 @​fold(i32 %a) nounwind {
%c = sdiv i32 %a, 2 ; [#uses=1]
ret i32 %c
}

by opt -std-compile-opts. These functions are not equivalent!

[llvmbot]

It all depends on whether getting a trap when dividing INT_MIN by
-1 is considered to be undefined or not...

[efriedma-quic]

Hmm, then maybe this is invalid... division by zero is considered to be undefined, so this probably falls into the same category. Would be nice if LangRef explicitly stated the expected behavior, though.

(Note that neither version actually faults: codegen rewrites a divide by -1 into a neg.)

[efriedma-quic]

Before I forget to note it, if I'm remembering correctly,
define i32 @​fold(i32 %a) {
%b = sub i32 0, %a
%c = sdiv i32 %b, -2
ret i32 %c
}
gets folded to the same thing, which is clearly wrong.

[llvmbot]

Is INT_MIN/(-2) defined? After all, you could say it is equal
to (INT_MIN/(-1))/2 which is not defined. What does the C standard
say about this one?

[efriedma-quic]

Hmm? Why would INT_MIN/-2 be undefined? It's in the range of numbers representable in a signed integer...

[efriedma-quic]

Marking invalid, since the behavior of INT_MIN/-1 is undefined. (But note the doc fix attached to Bug 1933.)