[gcc r14-9284] [PATCH] combine: Don't simplify paradoxical SUBREG on WORD_REGISTER_OPERATIONS [PR113010]

[Jeff Law]

https://gcc.gnu.org/g:24975a9195743e8eb4ca213f35b9221d4eeb6b59

commit r14-9284-g24975a9195743e8eb4ca213f35b9221d4eeb6b59
Author: Greg McGary <gkm@rivosinc.com>
Date:   Sun Mar 3 14:49:49 2024 -0700

    [PATCH] combine: Don't simplify paradoxical SUBREG on WORD_REGISTER_OPERATIONS [PR113010]
    
    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.
    
    gcc/
            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/testsuite
            * gcc.c-torture/execute/pr113010.c: New test.

Diff:
---
 gcc/combine.cc                                 | 15 +++++++++++++--
 gcc/testsuite/gcc.c-torture/execute/pr113010.c |  9 +++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

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..2c84333f5dc
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr113010.c
@@ -0,0 +1,9 @@
+int minus_1 = -1;
+
+int
+main ()
+{
+  if ((0, 0xffffffffull) >= minus_1)
+    __builtin_abort ();
+  return 0;
+}