diff --git a/gcc/config/loongarch/lasx.md b/gcc/config/loongarch/lasx.md
index f0f2dd08dd8..2e11f061202 100644
--- a/gcc/config/loongarch/lasx.md
+++ b/gcc/config/loongarch/lasx.md
@@ -3136,11 +3136,31 @@ (define_expand "copysign<mode>3"
 	  (match_operand:FLASX 1 "register_operand")))
    (set (match_dup 5)
 	(and:FLASX (match_dup 3)
-		   (match_operand:FLASX 2 "register_operand")))
+		   (match_operand:FLASX 2 "reg_or_vector_same_val_operand")))
    (set (match_operand:FLASX 0 "register_operand")
 	(ior:FLASX (match_dup 4) (match_dup 5)))]
   "ISA_HAS_LASX"
 {
+  /* copysign (x, -1) should instead be expanded as setting the sign
+     bit.  */
+  if (!REG_P (operands[2]))
+    {
+      rtx op2_elt = unwrap_const_vec_duplicate (operands[2]);
+      if (GET_CODE (op2_elt) == CONST_DOUBLE
+	  && real_isneg (CONST_DOUBLE_REAL_VALUE (op2_elt)))
+	{
+	  rtx n = GEN_INT (8 * GET_MODE_SIZE (<UNITMODE>mode) - 1);
+	  operands[0] = lowpart_subreg (<VIMODE256>mode, operands[0],
+					<MODE>mode);
+	  operands[1] = lowpart_subreg (<VIMODE256>mode, operands[1],
+					<MODE>mode);
+	  emit_insn (gen_lasx_xvbitseti_<lasxfmt> (operands[0],
+						   operands[1], n));
+	  DONE;
+	}
+    }
+
+  operands[2] = force_reg (<MODE>mode, operands[2]);
   operands[3] = loongarch_build_signbit_mask (<MODE>mode, 1, 0);
 
   operands[4] = gen_reg_rtx (<MODE>mode);
diff --git a/gcc/config/loongarch/lsx.md b/gcc/config/loongarch/lsx.md
index 55c7d79a030..8ea41c85b01 100644
--- a/gcc/config/loongarch/lsx.md
+++ b/gcc/config/loongarch/lsx.md
@@ -2873,11 +2873,31 @@ (define_expand "copysign<mode>3"
 	  (match_operand:FLSX 1 "register_operand")))
    (set (match_dup 5)
 	(and:FLSX (match_dup 3)
-		  (match_operand:FLSX 2 "register_operand")))
+		  (match_operand:FLSX 2 "reg_or_vector_same_val_operand")))
    (set (match_operand:FLSX 0 "register_operand")
 	(ior:FLSX (match_dup 4) (match_dup 5)))]
   "ISA_HAS_LSX"
 {
+  /* copysign (x, -1) should instead be expanded as setting the sign
+     bit.  */
+  if (!REG_P (operands[2]))
+    {
+      rtx op2_elt = unwrap_const_vec_duplicate (operands[2]);
+      if (GET_CODE (op2_elt) == CONST_DOUBLE
+	  && real_isneg (CONST_DOUBLE_REAL_VALUE (op2_elt)))
+	{
+	  rtx n = GEN_INT (8 * GET_MODE_SIZE (<UNITMODE>mode) - 1);
+	  operands[0] = lowpart_subreg (<VIMODE>mode, operands[0],
+					<MODE>mode);
+	  operands[1] = lowpart_subreg (<VIMODE>mode, operands[1],
+					<MODE>mode);
+	  emit_insn (gen_lsx_vbitseti_<lsxfmt> (operands[0], operands[1],
+						n));
+	  DONE;
+	}
+    }
+
+  operands[2] = force_reg (<MODE>mode, operands[2]);
   operands[3] = loongarch_build_signbit_mask (<MODE>mode, 1, 0);
 
   operands[4] = gen_reg_rtx (<MODE>mode);
diff --git a/gcc/testsuite/g++.target/loongarch/vect-copysign-negconst-run.C b/gcc/testsuite/g++.target/loongarch/vect-copysign-negconst-run.C
new file mode 100644
index 00000000000..d2d5d15c933
--- /dev/null
+++ b/gcc/testsuite/g++.target/loongarch/vect-copysign-negconst-run.C
@@ -0,0 +1,47 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -march=loongarch64 -mlasx -mno-strict-align" } */
+/* { dg-require-effective-target loongarch_asx_hw } */
+
+#include "vect-copysign-negconst.C"
+
+double d[] = {1.2, -3.4, -5.6, 7.8};
+float f[] = {1.2, -3.4, -5.6, 7.8, -9.0, -11.4, 51.4, 1919.810};
+
+double _abs(double x) { return __builtin_fabs (x); }
+float _abs(float x) { return __builtin_fabsf (x); }
+
+template <class T>
+void
+check (T *arr, T *orig, int len)
+{
+  for (int i = 0; i < len; i++)
+    {
+      if (arr[i] > 0)
+	__builtin_trap ();
+      if (_abs (arr[i]) != _abs (orig[i]))
+	__builtin_trap ();
+    }
+}
+
+int
+main()
+{
+  double test_d[4];
+  float test_f[8];
+
+  __builtin_memcpy (test_d, d, sizeof (test_d));
+  force_negative<2> (test_d);
+  check (test_d, d, 2);
+
+  __builtin_memcpy (test_d, d, sizeof (test_d));
+  force_negative<4> (test_d);
+  check (test_d, d, 4);
+
+  __builtin_memcpy (test_f, f, sizeof (test_f));
+  force_negative<4> (test_f);
+  check (test_f, f, 4);
+
+  __builtin_memcpy (test_f, f, sizeof (test_f));
+  force_negative<8> (test_f);
+  check (test_f, f, 8);
+}
diff --git a/gcc/testsuite/g++.target/loongarch/vect-copysign-negconst.C b/gcc/testsuite/g++.target/loongarch/vect-copysign-negconst.C
new file mode 100644
index 00000000000..5e8820d2bca
--- /dev/null
+++ b/gcc/testsuite/g++.target/loongarch/vect-copysign-negconst.C
@@ -0,0 +1,27 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=loongarch64 -mlasx -mno-strict-align" } */
+/* { dg-final { scan-assembler "\txvbitseti.*63" } } */
+/* { dg-final { scan-assembler "\txvbitseti.*31" } } */
+/* { dg-final { scan-assembler "\tvbitseti.*63" } } */
+/* { dg-final { scan-assembler "\tvbitseti.*31" } } */
+
+template <int N>
+__attribute__ ((noipa)) void
+force_negative (float *arr)
+{
+  for (int i = 0; i < N; i++)
+    arr[i] = __builtin_copysignf (arr[i], -2);
+}
+
+template <int N>
+__attribute__ ((noipa)) void
+force_negative (double *arr)
+{
+  for (int i = 0; i < N; i++)
+    arr[i] = __builtin_copysign (arr[i], -3);
+}
+
+template void force_negative<4>(float *);
+template void force_negative<8>(float *);
+template void force_negative<2>(double *);
+template void force_negative<4>(double *);
