URL:
<http://savannah.nongnu.org/bugs/?37026>
Summary: ABI documentation and 8-bit integer promotion of
function return
Project: AVR C Runtime Library
Submitted by: gjlayde
Submitted on: Sa 04 Aug 2012 12:48:57 GMT
Category: Documentation
Severity: 3 - Normal
Priority: 5 - Normal
Item Group: Documentation
Status: None
Percent Complete: 0%
Assigned to: None
Open/Closed: Open
Discussion Lock: Any
Release: 1.8.0
Fixed Release: None
_______________________________________________________
Details:
The current ABI documentation [1] reads:
----
* Return values: [...] 8-bit return values are zero/sign-extended to 16 bits
by the called function [...]
----
avr-gcc up to 4.2 actually performed that promotion. However, that promotion
was not per design, it was just a missed optimization in the compiler (and
thus is not suposed to work reliably) and since 4.3 (including respective
WinAVR releases) that promotion is no more performed.
There were off-list e-mail threads "AVR GCC 8-bit return values" from
2011-04-02 and "[off-list]: Updating avr-gcc ABI" from 2011-04-20 between
Eric, Jörg, Denis and me with the conclusion that the above clause shall be
dropped from the ABI.
The changed promotion behavior affects assembler code that calls C function
with an 8-bit integer return value and expects that the return value is
promoted to 16 bits.
Even the older versions <= 4.2 of the compiler that perform the promotion
won't make use of that: Suppose a caller that calls a function returning
8-bit, and the caller needs a 16-bit value. Then the caller performs that
extention explicitly and does not rely on the callee returning a 16-bit
integer. Sample code:
extern char a (void);
int b (void)
{
return a();
}
The ABI document should mention that the promotion performed by the compiler
changed with 4.2 -> 4.3 and the promotion up to 4.2 does not work reliably.
Moreover (and unrelated to promotion) the ABI should mention that T-flag in
SREG is used the same way like R0 aka. __tmp_reg__.
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