Re: [PATCH, AArch64] Use MOVN to generate 64-bit negative immediates where sensible

[Richard Earnshaw <rearnsha at arm dot com>]

On 08/05/14 18:36, Ian Bolton wrote:
> Hi,
> 
> It currently takes 4 instructions to generate certain immediates on
> AArch64 (unless we put them in the constant pool).
> 
> For example ...
> 
>   long long
>   ffffbeefcafebabe ()
>   {
>     return 0xFFFFBEEFCAFEBABEll;
>   }
> 
> leads to ...
> 
>   mov x0, 0x47806
>   mov x0, 0xcafe, lsl 16
>   mov x0, 0xbeef, lsl 32
>   orr x0, x0, -281474976710656
> 
> The above case is tackled in this patch by employing MOVN
> to generate the top 32-bits in a single instruction ...
> 
>   mov x0, -71536975282177
>   movk x0, 0xcafe, lsl 16
>   movk x0, 0xbabe, lsl 0
> 
> Note that where at least two half-words are 0xffff, existing
> code that does the immediate in two instructions is still used.)
> 
> Tested on standard gcc regressions and the attached test case.
> 
> OK for commit?

What about:

long long a()
{
  return 0x1234ffff56789abcll;
}

long long b()
{
  return 0x12345678ffff9abcll;
}

long long c()
{
  return 0x123456789abcffffll;
}

?

Surely these can also benefit from this sort of optimization, but it
looks as though you only handle the top 16 bits being set.

R.

> 
> Cheers,
> Ian
> 
> 
> 2014-05-08  Ian Bolton  <ian.bolton@arm.com>
> 
> gcc/
> 	* config/aarch64/aarch64.c (aarch64_expand_mov_immediate):
> 	Use MOVN when top-most half-word (and only that half-word)
> 	is 0xffff.
> gcc/testsuite/
> 	* gcc.target/aarch64/movn_1.c: New test.
> 
> 
> aarch64-movn-exploitation-patch-v5.txt
> 
> 
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index 43a83566..a8e504e 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -1177,6 +1177,18 @@ aarch64_expand_mov_immediate (rtx dest, rtx imm)
>  	}
>      }
>  
> +  /* Look for case where upper 16 bits are set, so we can use MOVN.  */
> +  if ((val & 0xffff000000000000ll) == 0xffff000000000000ll)
> +    {
> +      emit_insn (gen_rtx_SET (VOIDmode, dest,
> +			      GEN_INT (~ (~val & (0xffffll << 32)))));
> +      emit_insn (gen_insv_immdi (dest, GEN_INT (16),
> +				 GEN_INT ((val >> 16) & 0xffff)));
> +      emit_insn (gen_insv_immdi (dest, GEN_INT (0),
> +				 GEN_INT (val & 0xffff)));
> +      return;
> +    }
> +
>   simple_sequence:
>    first = true;
>    mask = 0xffff;
> diff --git a/gcc/testsuite/gcc.target/aarch64/movn_1.c b/gcc/testsuite/gcc.target/aarch64/movn_1.c
> new file mode 100644
> index 0000000..cc11ade
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/movn_1.c
> @@ -0,0 +1,27 @@
> +/* { dg-do run } */
> +/* { dg-options "-O2 -fno-inline --save-temps" } */
> +
> +extern void abort (void);
> +
> +long long
> +foo ()
> +{
> +  /* { dg-final { scan-assembler "mov\tx\[0-9\]+, -71536975282177" } } */
> +  return 0xffffbeefcafebabell;
> +}
> +
> +long long
> +merge4 (int a, int b, int c, int d)
> +{
> +  return ((long long) a << 48 | (long long) b << 32
> +	  | (long long) c << 16 | (long long) d);
> +}
> +
> +int main ()
> +{
> +  if (foo () != merge4 (0xffff, 0xbeef, 0xcafe, 0xbabe))
> +    abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { cleanup-saved-temps } } */
>