duplicate "extractvalue" not eliminated

[llvmbot]

Bugzilla Link	3623
Resolution	FIXED
Resolved on	Dec 02, 2010 01:37
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@efriedma-quic,@nlewycky

Extended Description

In this .ll file:

define double @​zsqr({ double, double } %a) {
%X = extractvalue { double, double } %a, 0
%Y = extractvalue { double, double } %a, 0
%Z = mul double %X, %Y
ret double %Z
}

no optimization pass in -std-compile-opts folds %X and %Y.

[lattner]

GVN isn't handling extractvalue

[llvmbot]

Does this really need GVN? Why can't instcombine just detect that these two are equivalent because they have the same operands like it does for add, mul, etc?

Recall that "insertvalue" can't mutate %a, it can only return a new SSA variable. "extractvalue" isn't like load/store in that sense.

[efriedma-quic]

Hmm? instcombine doesn't attempt to do structural equivalence. This needs GVN in the same sense combining (a2/3)+(a2/3) to 2*(a*2/3) needs GVN.

[nlewycky]

Incidentally instcombine will handle that particular example, but you're right in general. I thought instcombine called I1->isIdenticalTo(I2) and folded, but apparently I'm dreaming again. :)

GVN it is, then!

[lattner]

GVN gets this now.