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From: "alex at sunopti dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug d/97843] Bad code gen when concatenating to array
Date: Mon, 16 Nov 2020 12:16:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97843

--- Comment #2 from Alex <alex at sunopti dot com> ---
I agree that the order of evaluation of operands is undefined and writing c=
ode
that depends on that order would not be reliable. In this case it's the
execution of the assign expression happening before all the operands have b=
een
evaluated that is the problem.

The arithmetic equivalent would be for:
X +=3D 4/2
To be produce:
Immediate load Register with 4
Add register with 4 in it to x
Divide register with 4 in it by 2
Resulting in x being increased by 4 instead of 2

10.2.3 Binary expressions EXCEPT for AssignExpression are left to right

10.2.7 says operand order is undefined

Nowhere does the spec say that the assignment operator has to happen after
operand evaluation. I think this is a hole in the spec.=