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[avr-gcc-list] char to int promotion in bitwise operators

From: Andrew Zabolotny
Subject: [avr-gcc-list] char to int promotion in bitwise operators
Date: Fri, 21 Aug 2009 02:27:25 +0400


I've been looking at the compiled .S file for my library and have
noticed that the code is substantially larger than could be because
every my check for bitflags, like in this example:

#define FLAG_A 0x01
#define FLAG_B 0x02


if (flags & FLAG_A) ...
if (flags & FLAG_B) ...

gcc will extend both 'flags' and FLAG_A to a 16-bit integer type and
then do a 16-bit 'and' and compare. I recalled that I've seen something
about this in avr-libc documentation, and indeed I found the section:

11.21 Why does the compiler compile an 8-bit operation that uses
      bitwise operators into a 16-bit operation in assembly?

However, the solution in that section does not help here anyway. If I
modify those if's to look like:

if (((unsigned char)flags) & ((unsigned char)FLAG_A) ...

gcc will anyway expand both operands to int first :-( Even the
following cumbersome example will result in everything being expanded
(and even compared!) as 16-bit integers:

if ((unsigned char)(((unsigned char)a) & ((unsigned char)FLAG)) !=
(unsigned char)0) ...

Is there a way to force the compiler to use 8-bit integers for bitwise
operators? The only way I found so far is to use the -mint8 switch, but
libc docs says that this option is not really supported by libc.


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