Hi,
On Tue, 8 Dec 2009, H.J. Lu wrote:
> Both icc and gcc generate:
>
> [...@gnu-26 pr42324]$ cat b4.c
> extern unsigned int bartmp;
>
> void foo(_Bool bar)
> {
> bartmp = bar;
> }
> [...@gnu-26 pr42324]$ /usr/gcc-4.4/bin/gcc -O2 b4.c -S
> [...@gnu-26 pr42324]$ cat b4.s
> .file "b4.c"
> .text
> .p2align 4,,15
> .globl foo
> .type foo, @function
> foo:
> .LFB2:
> movzbl %dil, %edi
> movl %edi, bartmp(%rip)
> ret
Yes, so they expect the upper bits (at least 1-7) cleared on the caller
side. And if they do that, somebody needs to make this guarantee which
only the ABI can.
> We should just drop
>
> ---
> When a value of type _Bool is passed in a register or on the stack,
> the upper 63 bits of the eightbyte shall be zero.
> ---
If anything we can only change it to say something less strict ...
> from psABI. Since _Bool has one byte in size with values of 0 and 1.
> Compilers have to clear upper 7 bits in one byte.
... because this part can only be guaranteed by the ABI. Without the
above language a compiler would be free to implement any non-zero byte as
true for parameter passing without violating the ABI.
Ciao,
Michael.
