Re: [PATCH] LoongArch: Fix wrong code from bstrpick split

[Lulu Cheng]

在 2025/9/12 下午10:36, Xi Ruoyao 写道:

After late-combine is added, split1 can see an input like

     (insn 56 55 169 5
       (set (reg/v:DI 87 [ n ])
         (ior:DI (and:DI (reg/v:DI 87 [ n ])
                         (const_int 281474976710655 [0xffffffffffff]))
                 (and:DI (reg:DI 131 [ _45 ])
                         (const_int -281474976710656 [0xffff000000000000]))))
       "pr121906.c":22:8 108 {*bstrins_di_for_ior_mask}
       (nil))

And the splitter ends up emitting

     (insn 184 55 185 5
       (set (reg/v:DI 87 [ n ])
            (reg:DI 131 [ _45 ]))
       "pr121906.c":22:8 -1
       (nil))
     (insn 185 184 169 5
       (set (zero_extract:DI (reg/v:DI 87 [ n ])
                             (const_int 48 [0x30])
                             (const_int 0 [0]))
            (reg/v:DI 87 [ n ]))
       "pr121906.c":22:8 -1
       (nil))

which obviously lost everything in r87, instead of retaining its lower
bits as we expect.  It's because the splitter didn't anticipate the
output register may be one of the input registers.

        PR target/121906

gcc/

        * config/loongarch/loongarch.md (*bstrins_<mode>_for_ior_mask):
        Always create a new pseudo for the input register of the bstrins
        instruction.

gcc/testsuite/

        * gcc.target/loongarch/pr121906.c: New test.
---

Bootstrapped and regtested on loongarch64-linux-gnu.  Ok for trunk and
releases/gcc-15?

Ok!  Thanks!

  gcc/config/loongarch/loongarch.md             | 14 ++++-----
  gcc/testsuite/gcc.target/loongarch/pr121906.c | 31 +++++++++++++++++++
  2 files changed, 38 insertions(+), 7 deletions(-)
  create mode 100644 gcc/testsuite/gcc.target/loongarch/pr121906.c

diff --git a/gcc/config/loongarch/loongarch.md 
b/gcc/config/loongarch/loongarch.md
index 32ef9809b10..f42dc102d10 100644
--- a/gcc/config/loongarch/loongarch.md
+++ b/gcc/config/loongarch/loongarch.md
@@ -1619,13 +1619,13 @@ (define_insn_and_split "*bstrins_<mode>_for_ior_mask"
      operands[2] = GEN_INT (len);
      operands[4] = GEN_INT (lo);

- if (lo)

-      {
-       rtx tmp = gen_reg_rtx (<MODE>mode);
-       emit_move_insn (tmp, gen_rtx_ASHIFTRT(<MODE>mode, operands[3],
-                                             GEN_INT (lo)));
-       operands[3] = tmp;
-      }
+    /* Use a new pseudo register even if lo == 0 or we'll wreck havoc
+       when operands[0] is same as operands[3].  See PR 121906.  */
+    rtx tmp = gen_reg_rtx (<MODE>mode);
+    rtx val = lo ? gen_rtx_ASHIFTRT (<MODE>mode, operands[3], GEN_INT (lo))
+                : operands[3];
+    emit_move_insn (tmp, val);
+    operands[3] = tmp;
    })

;; We always avoid the shift operation in bstrins_<mode>_for_ior_mask

diff --git a/gcc/testsuite/gcc.target/loongarch/pr121906.c 
b/gcc/testsuite/gcc.target/loongarch/pr121906.c
new file mode 100644
index 00000000000..b4fde5f0c85
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/pr121906.c
@@ -0,0 +1,31 @@
+/* PR target/121906 */
+/* { dg-do run } */
+/* { dg-options "-O2 -mno-lsx" } */
+
+typedef unsigned short u16;
+typedef unsigned long u64;
+typedef u16 v4hi __attribute__ ((vector_size (8)));
+typedef u16 v8hi __attribute__ ((vector_size (16)));
+
+u64 d;
+int e, i;
+u16 x;
+
+int
+main ()
+{
+  v4hi n = { 1 };
+  u64 *o = &d;
+p:
+  asm goto ("" : : : : q);
+  n[3] = (-(v8hi){ 0, 0, 0, 0, x })[7];
+  for (; e >= 0; e--)
+    {
+      *o = n[0];
+      if (i)
+        goto p;
+    q:
+    }
+  if (d != 1)
+    __builtin_trap ();
+}