Extended Description

Before r239309, this works:

[hjl@gnu-32 bin]$ cat x.c
void
foo (int p)
{
register int reg asm("r8") = p;
asm volatile("# %0" : : "r" (reg));
}
[hjl@gnu-32 bin]$ ./clang -S -O2 x.c
[hjl@gnu-32 bin]$ cat x.s
.text
.file "x.c"
.globl foo
.align 16, 0x90
.type foo,@function
foo: # @​foo
.cfi_startproc

BB#0:

movl	%edi, %r8d
#APP

#NO_APP
retq

.Lfunc_end0:
.size foo, .Lfunc_end0-foo
.cfi_endproc

Now, I got

[hjl@gnu-6 bin]$ ./clang -S -O2 x.c
x.c:5:24: error: couldn't allocate input reg for constraint '{r8}'
asm volatile("# %0" : : "r" (reg));
^
1 error generated.
[hjl@gnu-6 bin]$

If I use "r8d", I get

[hjl@gnu-6 bin]$ cat x.c
void
foo (int p)
{
register int reg asm("r8d") = p;
asm volatile("# %0" : : "r" (reg));
}
[hjl@gnu-6 bin]$ ./clang -S -O2 x.c
x.c:4:28: error: unknown register name 'r8d' in asm
register int reg asm("r8d") = p;
^
1 error generated.
[hjl@gnu-6 bin]$