[PATCH] combine: Don't simplify paradoxical SUBREG on WORD_REGISTER_OPERATIONS [PR113010]

[PATCH] combine: Don't simplify paradoxical SUBREG on WORD_REGISTER_OPERATIONS [PR113010]

[Greg McGary]

The sign-bit-copies of a sign-extending load cannot be known until runtime on
WORD_REGISTER_OPERATIONS targets, except in the case of a zero-extending MEM
load.  See the fix for PR112758.

2024-02-22  Greg McGary  <gkm@rivosinc.com>

        PR rtl-optimization/113010
	* combine.cc (simplify_comparison): Simplify a SUBREG on
	  WORD_REGISTER_OPERATIONS targets only if it is a zero-extending
	  MEM load.

	* gcc.c-torture/execute/pr113010.c: New test.
---
 gcc/combine.cc                                 | 15 +++++++++++++--
 gcc/testsuite/gcc.c-torture/execute/pr113010.c |  9 +++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/gcc.c-torture/execute/pr113010.c

diff --git a/gcc/combine.cc b/gcc/combine.cc
index 76543d85b7c..b09200d757e 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -12550,9 +12550,20 @@ simplify_comparison (enum rtx_code code, rtx *pop0, rtx *pop1)
 	    }
 
 	  /* If the inner mode is narrower and we are extracting the low part,
-	     we can treat the SUBREG as if it were a ZERO_EXTEND.  */
+	     we can treat the SUBREG as if it were a ZERO_EXTEND ...  */
 	  if (paradoxical_subreg_p (op0))
-	    ;
+	    {
+	      if (WORD_REGISTER_OPERATIONS
+		  && GET_MODE_PRECISION (inner_mode) < BITS_PER_WORD
+		  /* On WORD_REGISTER_OPERATIONS targets the bits
+		     beyond sub_mode aren't considered undefined,
+		     so optimize only if it is a MEM load when MEM loads
+		     zero extend, because then the upper bits are all zero.  */
+		  && !(MEM_P (SUBREG_REG (op0))
+		       && load_extend_op (inner_mode) == ZERO_EXTEND))
+		break;
+	      /* FALLTHROUGH to case ZERO_EXTEND */
+	    }
 	  else if (subreg_lowpart_p (op0)
 		   && GET_MODE_CLASS (mode) == MODE_INT
 		   && is_int_mode (GET_MODE (SUBREG_REG (op0)), &inner_mode)
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr113010.c b/gcc/testsuite/gcc.c-torture/execute/pr113010.c
new file mode 100644
index 00000000000..a95c613c1df
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr113010.c
@@ -0,0 +1,9 @@
+int minus_1 = -1;
+
+int
+main ()
+{
+  if ((0, 0xfffffffful) >= minus_1)
+    __builtin_abort ();
+  return 0;
+}
-- 
2.34.1

Re: [PATCH] combine: Don't simplify paradoxical SUBREG on WORD_REGISTER_OPERATIONS [PR113010]

[Greg McGary]

On 2/26/24 5:17 PM, Greg McGary wrote:
> diff --git a/gcc/testsuite/gcc.c-torture/execute/pr113010.c b/gcc/testsuite/gcc.c-torture/execute/pr113010.c
> new file mode 100644
> index 00000000000..a95c613c1df
> --- /dev/null
> +++ b/gcc/testsuite/gcc.c-torture/execute/pr113010.c
> @@ -0,0 +1,9 @@
> +int minus_1 = -1;
> +
> +int
> +main ()
> +{
> +  if ((0, 0xfffffffful) >= minus_1)
> +    __builtin_abort ();
> +  return 0;
> +}


Note that this is a stale version of the testcase. The constant needs to be
long long 0xffffffffull for the sake of 32-bit machines, such as ARM.

G

Re: [PATCH] combine: Don't simplify paradoxical SUBREG on WORD_REGISTER_OPERATIONS [PR113010]

[Jeff Law]

On 2/26/24 17:17, Greg McGary wrote:
> The sign-bit-copies of a sign-extending load cannot be known until runtime on
> WORD_REGISTER_OPERATIONS targets, except in the case of a zero-extending MEM
> load.  See the fix for PR112758.
> 
> 2024-02-22  Greg McGary  <gkm@rivosinc.com>
> 
>          PR rtl-optimization/113010
> 	* combine.cc (simplify_comparison): Simplify a SUBREG on
> 	  WORD_REGISTER_OPERATIONS targets only if it is a zero-extending
> 	  MEM load.
> 
> 	* gcc.c-torture/execute/pr113010.c: New test.
I think this is fine for the trunk.  I'll do some final testing on it 
tomorrow.

Jeff

Re: [PATCH] combine: Don't simplify paradoxical SUBREG on WORD_REGISTER_OPERATIONS [PR113010]

[Rainer Orth]

Hi Jeff,

> On 2/26/24 17:17, Greg McGary wrote:
>> The sign-bit-copies of a sign-extending load cannot be known until runtime on
>> WORD_REGISTER_OPERATIONS targets, except in the case of a zero-extending MEM
>> load.  See the fix for PR112758.
>> 2024-02-22  Greg McGary  <gkm@rivosinc.com>
>>          PR rtl-optimization/113010
>> 	* combine.cc (simplify_comparison): Simplify a SUBREG on
>> 	  WORD_REGISTER_OPERATIONS targets only if it is a zero-extending
>> 	  MEM load.
>> 	* gcc.c-torture/execute/pr113010.c: New test.
> I think this is fine for the trunk.  I'll do some final testing on it
> tomorrow.

unfortunately, the patch broke Solaris/SPARC bootstrap
(sparc-sun-solaris2.11):

/vol/gcc/src/hg/master/local/gcc/combine.cc: In function 'rtx_code simplify_comparison(rtx_code, rtx_def**, rtx_def**)':
/vol/gcc/src/hg/master/local/gcc/combine.cc:12101:25: error: '*(unsigned int*)((char*)&inner_mode + offsetof(scalar_int_mode, scalar_int_mode::m_mode))' may be used uninitialized [-Werror=maybe-uninitialized]
12101 |   scalar_int_mode mode, inner_mode, tmode;
      |                         ^~~~~~~~~~

	Rainer

-- 
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University

[PATCH] combine: Fix recent WORD_REGISTER_OPERATIONS check [PR113010]

[Jakub Jelinek]

On Mon, Mar 04, 2024 at 05:18:39PM +0100, Rainer Orth wrote:
> > On 2/26/24 17:17, Greg McGary wrote:
> >> The sign-bit-copies of a sign-extending load cannot be known until runtime on
> >> WORD_REGISTER_OPERATIONS targets, except in the case of a zero-extending MEM
> >> load.  See the fix for PR112758.
> >> 2024-02-22  Greg McGary  <gkm@rivosinc.com>
> >>          PR rtl-optimization/113010
> >> 	* combine.cc (simplify_comparison): Simplify a SUBREG on
> >> 	  WORD_REGISTER_OPERATIONS targets only if it is a zero-extending
> >> 	  MEM load.
> >> 	* gcc.c-torture/execute/pr113010.c: New test.
> > I think this is fine for the trunk.  I'll do some final testing on it
> > tomorrow.
> 
> unfortunately, the patch broke Solaris/SPARC bootstrap
> (sparc-sun-solaris2.11):
> 
> /vol/gcc/src/hg/master/local/gcc/combine.cc: In function 'rtx_code simplify_comparison(rtx_code, rtx_def**, rtx_def**)':
> /vol/gcc/src/hg/master/local/gcc/combine.cc:12101:25: error: '*(unsigned int*)((char*)&inner_mode + offsetof(scalar_int_mode, scalar_int_mode::m_mode))' may be used uninitialized [-Werror=maybe-uninitialized]
> 12101 |   scalar_int_mode mode, inner_mode, tmode;
>       |                         ^~~~~~~~~~

I don't see how it could ever work properly, inner_mode in that spot is
just uninitialized.

I think we shouldn't worry about paradoxical subregs of non-scalar_int_mode
REGs/MEMs and for the scalar_int_mode ones should initialize inner_mode
before we use it.
Another option would be to use
maybe_lt (GET_MODE_PRECISION (GET_MODE (SUBREG_REG (op0))), BITS_PER_WORD)
and
load_extend_op (GET_MODE (SUBREG_REG (op0))) == ZERO_EXTEND,
or set machine_mode smode = GET_MODE (SUBREG_REG (op0)); and use it in
those two spots.

2024-03-04  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/113010
	* combine.cc (simplify_comparison): Guard the
	WORD_REGISTER_OPERATIONS check on scalar_int_mode of SUBREG_REG
	and initialize inner_mode.

--- gcc/combine.cc.jj	2024-03-04 10:01:21.054937316 +0100
+++ gcc/combine.cc	2024-03-04 17:40:51.556052647 +0100
@@ -12554,6 +12554,8 @@ simplify_comparison (enum rtx_code code,
 	  if (paradoxical_subreg_p (op0))
 	    {
 	      if (WORD_REGISTER_OPERATIONS
+		  && is_a <scalar_int_mode> (GET_MODE (SUBREG_REG (op0)),
+					     &inner_mode)
 		  && GET_MODE_PRECISION (inner_mode) < BITS_PER_WORD
 		  /* On WORD_REGISTER_OPERATIONS targets the bits
 		     beyond sub_mode aren't considered undefined,


	Jakub

Re: [PATCH] combine: Fix recent WORD_REGISTER_OPERATIONS check [PR113010]

[Jeff Law]

On 3/4/24 09:49, Jakub Jelinek wrote:
> On Mon, Mar 04, 2024 at 05:18:39PM +0100, Rainer Orth wrote:
>>> On 2/26/24 17:17, Greg McGary wrote:
>>>> The sign-bit-copies of a sign-extending load cannot be known until runtime on
>>>> WORD_REGISTER_OPERATIONS targets, except in the case of a zero-extending MEM
>>>> load.  See the fix for PR112758.
>>>> 2024-02-22  Greg McGary  <gkm@rivosinc.com>
>>>>           PR rtl-optimization/113010
>>>> 	* combine.cc (simplify_comparison): Simplify a SUBREG on
>>>> 	  WORD_REGISTER_OPERATIONS targets only if it is a zero-extending
>>>> 	  MEM load.
>>>> 	* gcc.c-torture/execute/pr113010.c: New test.
>>> I think this is fine for the trunk.  I'll do some final testing on it
>>> tomorrow.
>>
>> unfortunately, the patch broke Solaris/SPARC bootstrap
>> (sparc-sun-solaris2.11):
>>
>> /vol/gcc/src/hg/master/local/gcc/combine.cc: In function 'rtx_code simplify_comparison(rtx_code, rtx_def**, rtx_def**)':
>> /vol/gcc/src/hg/master/local/gcc/combine.cc:12101:25: error: '*(unsigned int*)((char*)&inner_mode + offsetof(scalar_int_mode, scalar_int_mode::m_mode))' may be used uninitialized [-Werror=maybe-uninitialized]
>> 12101 |   scalar_int_mode mode, inner_mode, tmode;
>>        |                         ^~~~~~~~~~
> 
> I don't see how it could ever work properly, inner_mode in that spot is
> just uninitialized.
> 
> I think we shouldn't worry about paradoxical subregs of non-scalar_int_mode
> REGs/MEMs and for the scalar_int_mode ones should initialize inner_mode
> before we use it.
> Another option would be to use
> maybe_lt (GET_MODE_PRECISION (GET_MODE (SUBREG_REG (op0))), BITS_PER_WORD)
> and
> load_extend_op (GET_MODE (SUBREG_REG (op0))) == ZERO_EXTEND,
> or set machine_mode smode = GET_MODE (SUBREG_REG (op0)); and use it in
> those two spots.
> 
> 2024-03-04  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR rtl-optimization/113010
> 	* combine.cc (simplify_comparison): Guard the
> 	WORD_REGISTER_OPERATIONS check on scalar_int_mode of SUBREG_REG
> 	and initialize inner_mode.
Egad.  Sorry.  OK for the trunk.  Thanks for picking this up.  Got 
distracted by an internal issue.

jeff