Re: [PATCH v2] improve folding of expressions that move a single bit around

[Richard Biener <rguenther at suse dot de>]

On Thu, 1 Dec 2016, Paolo Bonzini wrote:

> In code like the following from KVM:
> 
>         /* it is a read fault? */
>         error_code = (exit_qualification << 2) & PFERR_FETCH_MASK;
> 
> it would be nicer to write
> 
>         /* it is a read fault? */
>         error_code = (exit_qualification & VMX_EPT_READ_FAULT_MASK) ? PFERR_FETCH_MASK : 0;
> 
> instead of having to know the difference between the positions of the
> source and destination bits.  LLVM catches the latter just fine (which
> is why I am sending this in stage 3...), but GCC does not, so this
> patch adds two patterns to catch it.
> 
> The combine.c hunk of v1 has been committed already.
> 
> Bootstrapped/regtested x86_64-pc-linux-gnu, ok?

Ok.

Thanks,
Richard.

> Paolo
> 
> 2016-11-26  Paolo Bonzini  <bonzini@gnu.org>
> 
> 	* match.pd: Simplify X ? C : 0 where C is a power of 2 and
> 	X tests a single bit.
> 
> 2016-11-26  Paolo Bonzini  <bonzini@gnu.org>
> 
> 	* gcc.dg/fold-and-lshift.c, gcc.dg/fold-and-rshift-1.c,
> 	gcc.dg/fold-and-rshift-2.c: New testcases.
> 
> Index: match.pd
> ===================================================================
> --- match.pd	(revision 242916)
> +++ match.pd	(working copy)
> @@ -2630,6 +2630,21 @@
>    (cmp (bit_and@2 @0 integer_pow2p@1) @1)
>    (icmp @2 { build_zero_cst (TREE_TYPE (@0)); })))
>   
> +/* If we have (A & C) != 0 ? D : 0 where C and D are powers of 2,
> +   convert this into a shift followed by ANDing with D.  */
> +(simplify
> + (cond
> +  (ne (bit_and @0 integer_pow2p@1) integer_zerop)
> +  integer_pow2p@2 integer_zerop)
> + (with {
> +    int shift = wi::exact_log2 (@2) - wi::exact_log2 (@1);
> +  }
> +  (if (shift > 0)
> +   (bit_and
> +    (lshift (convert @0) { build_int_cst (integer_type_node, shift); }) @2)
> +   (bit_and
> +    (convert (rshift @0 { build_int_cst (integer_type_node, -shift); })) @2))))
> +
>  /* If we have (A & C) != 0 where C is the sign bit of A, convert
>     this into A < 0.  Similarly for (A & C) == 0 into A >= 0.  */
>  (for cmp (eq ne)
> @@ -2644,6 +2659,19 @@
>     (with { tree stype = signed_type_for (TREE_TYPE (@0)); }
>      (ncmp (convert:stype @0) { build_zero_cst (stype); })))))
>  
> +/* If we have A < 0 ? C : 0 where C is a power of 2, convert
> +   this into a right shift followed by ANDing with C.  */
> +(simplify
> + (cond
> +  (lt @0 integer_zerop)
> +  integer_pow2p@1 integer_zerop)
> + (with {
> +    int shift = element_precision (@0) - wi::exact_log2 (@1) - 1;
> +  }
> +  (bit_and
> +   (convert (rshift @0 { build_int_cst (integer_type_node, shift); }))
> +   @1)))
> +
>  /* When the addresses are not directly of decls compare base and offset.
>     This implements some remaining parts of fold_comparison address
>     comparisons but still no complete part of it.  Still it is good
> Index: testsuite/gcc.dg/fold-and-lshift.c
> ===================================================================
> --- testsuite/gcc.dg/fold-and-lshift.c	(revision 0)
> +++ testsuite/gcc.dg/fold-and-lshift.c	(working copy)
> @@ -0,0 +1,35 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O -fdump-tree-original" } */
> +
> +int f(int x)
> +{
> +	return (x << 2) & 128;
> +}
> +
> +int g(int x)
> +{
> +	return !!(x & 32) << 7;
> +}
> +
> +int h(int x)
> +{
> +	return ((x >> 5) & 1) << 7;
> +}
> +
> +int i(int x)
> +{
> +	return (x & 32) >> 5 << 7;
> +}
> +
> +int j(int x)
> +{
> +	return ((x >> 5) & 1) ? 128 : 0;
> +}
> +
> +int k(int x)
> +{
> +	return (x & 32) ? 128 : 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-not " \\? " "original" } } */
> +/* { dg-final { scan-assembler-not "sarl" { target i?86-*-* x86_64-*-* } } }" */
> Index: testsuite/gcc.dg/fold-and-rshift-1.c
> ===================================================================
> --- testsuite/gcc.dg/fold-and-rshift-1.c	(revision 0)
> +++ testsuite/gcc.dg/fold-and-rshift-1.c	(working copy)
> @@ -0,0 +1,35 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O -fdump-tree-original" } */
> +
> +int f(int x)
> +{
> +	return (x >> 2) & 128;
> +}
> +
> +int g(int x)
> +{
> +	return !!(x & 512) << 7;
> +}
> +
> +int h(int x)
> +{
> +	return ((x >> 9) & 1) << 7;
> +}
> +
> +int i(int x)
> +{
> +	return (x & 512) >> 9 << 7;
> +}
> +
> +int j(int x)
> +{
> +	return ((x >> 9) & 1) ? 128 : 0;
> +}
> +
> +int k(int x)
> +{
> +	return (x & 512) ? 128 : 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-not " \\? " "original" } } */
> +/* { dg-final { scan-assembler-not "sall" { target i?86-*-* x86_64-*-* } } }" */
> Index: testsuite/gcc.dg/fold-and-rshift-2.c
> ===================================================================
> --- testsuite/gcc.dg/fold-and-rshift-2.c	(revision 0)
> +++ testsuite/gcc.dg/fold-and-rshift-2.c	(working copy)
> @@ -0,0 +1,25 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O -fdump-tree-original" } */
> +
> +unsigned f(unsigned x)
> +{
> +	return (x >> 29) & 32;
> +}
> +
> +unsigned g(unsigned x)
> +{
> +	return !!(x & 0x80000000) << 5;
> +}
> +
> +unsigned j(unsigned x)
> +{
> +	return ((x >> 31) & 1) ? 32 : 0;
> +}
> +
> +unsigned k(unsigned x)
> +{
> +	return (x & 0x80000000) ? 32 : 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-not " \\? " "original" } } */
> +/* { dg-final { scan-assembler-not "sall" { target i?86-*-* x86_64-*-* } } }" */
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)