David Hagood schrieb:
On Sun, 2009-06-07 at 12:00 +0200, Georg-Johann Lay wrote:
in the test cases
register unsigned char reg2 asm ("r2");
As gcc 3.4.6 behaves as expected (e.g. in bar, the access to reg2
remains in the loop), I wonder if this aggressive optimization is a bug?
IMO that is a bug because reg2 is a fixed register and should
therefore be treated just like volatile. Unfortunately, gcc has
nothing like REG_VOLATILE_P to tag a reg being volatile, but as I
said global fixed registers should be treated that way.
Try adding a volatile to the variable:
register volatile unsigned char reg2 asm ("r2");
That has no effect. Note that register is not a mem. Older versions of
gcc reported a warning on volatile register like "effect is not what you
might expect". Moreover, gcc cannot track volatile registers.
rtl.h reads:
/* 1 in a MEM or ASM_OPERANDS expression if the memory reference is
volatile.
1 in an INSN, CALL_INSN, JUMP_INSN, CODE_LABEL, BARRIER, or NOTE
if it has been deleted.
1 in a REG expression if corresponds to a variable declared by the
user,
0 for an internally generated temporary.
1 in a SUBREG with a negative value.
1 in a LABEL_REF, REG_LABEL_TARGET or REG_LABEL_OPERAND note for a
non-local label.
In a SYMBOL_REF, this flag is used for machine-specific purposes. */
unsigned int volatil : 1;
What gcc knows is that some registers are or may be fixed, so it should
care about that.