I have just noticed a bug in the latest release of GDC that
targets x86-64. For example GDC 12.3 and above versions too,
running on X86-64, targeting self. This was built with:
-O3 -frelease -march=alderlake
Generated code is:
je L1
jbe L1
…
…
L1: ret
The probably reason for this is my use of the GDC built in for
indicating whether conditional jumps are likely or unlikely to be
taken. I wrote a trivial routine likely() and unlikely() and used
this as follows:
public
T pow(T, exp_ui_t )( in T x, in exp_ui_t p ) pure @safe nothrow
@nogc
if ( is ( exp_ui_t == ulong ) || is ( exp_ui_t == uint ) )
in {
static assert( is ( typeof( x * x ) ) );
assert( p >= 0 );
}
out ( ret ) {
assert( ( p == 0 && ret == 1 ) || !( p == 0 ) );
}
do
{
if ( unlikely( p == 0 ) ) return 1;
if ( unlikely( p == 1 ) ) return x;
/*
if ( unlikely( x == 0 ) ) // fast-path opt, unnecessary
return x;
if ( unlikely( x == 1 ) ) // fast-path opt, unnecessary
return x;
*/
T s = x;
T v = 1;
for ( exp_ui_t i = p; i > 1; i >>= 1 )
{
v = ( i & 0x1 ) ? s * v : v;
s = s * s;
}
//assert( p > 1 && pow( x, p ) == ( p > 1 ? x * pow( x, p-1) :
1) );
return v * s;
}
pragma( inline, true )
private
bool builtin_expect()( in bool test_cond, in bool expected_cond )
pure nothrow @safe @nogc
{
version ( LDC )
{// ldc.intrinsics.llvm_expect - didi not seem to work when
tested in LDC 1.22
import ldc.intrinsics : llvm_expect;
return cast(bool) llvm_expect( test_cond, expected_cond );
}
version ( GDC )
{
import gcc.builtins : __builtin_expect;
return cast(bool) __builtin_expect( test_cond, expected_cond
);
}
return test_cond;
}
pragma( inline, true )
public
bool likely()( in bool test_cond ) pure nothrow @safe @nogc
/* Returns test_cond which makes it convenient to do assert(
unlikely() )
* Also emulates builtin_expect's return behaviour, by returning
the argument
*/ {
return builtin_expect( test_cond, true );
}
pragma( inline, true )
public
bool unlikely()( in bool test_cond ) pure nothrow @safe @nogc
/* Returns test_cond which makes it convenient to do assert(
unlikely() )
* Also emulates builtin_expect's return behaviour, by returning
the argument
*/
{
return builtin_expect( test_cond, false );
}
// ~~~ module likely - end.
====
This is not the whole of this .d file, I can of course give you
the whole lot if you desire. I inspected the result in Matt
Godbolt’s compiler explorer website godbolt.org.
An aside: LDC:: I need to look at LDC’s llvm_expect to see if it
is controlling the branches the way I wish. Does anyone know if
llvm_expect has any problems?
