[gcc r17-3520] RISC-V: Fix sub-Pmode index in vec_set/vec_extract [PR126873]

https://gcc.gnu.org/g:1a29fdbdcb6775cd93bfc23426bf4e8b6b0475d2

commit r17-3520-g1a29fdbdcb6775cd93bfc23426bf4e8b6b0475d2
Author: Jin Ma <[email protected]>
Date:   Tue Aug 18 23:31:24 2026 +0800

    RISC-V: Fix sub-Pmode index in vec_set/vec_extract [PR126873]
    
    When gimple-isel lowers a variable-index element access on a
    fixed-length vector into .VEC_SET or .VEC_EXTRACT, the index may be
    narrower than Pmode.  The vec_set and vec_extract expanders used
    gen_lowpart to convert it to Pmode, which only reinterprets the
    register and drops the source-level truncation.  The full 64-bit
    value is then used as the slide amount of vslideup/vslidedown and
    wrong code is generated.
    
    Fix this by zero-extending a sub-Pmode index with convert_to_mode
    instead.
    
            PR target/126873
    
    gcc/ChangeLog:
    
            * config/riscv/autovec.md (vec_set<mode>): Zero-extend a
            sub-Pmode index to Pmode.
            (vec_extract<mode><vel>): Likewise.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.target/riscv/pr126873.c: New test.
    
    Reported-by: XChy <[email protected]>
    Signed-off-by: Jin Ma <[email protected]>

Diff:
---
 gcc/config/riscv/autovec.md               |  4 ++--
 gcc/testsuite/gcc.target/riscv/pr126873.c | 37 +++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/gcc/config/riscv/autovec.md b/gcc/config/riscv/autovec.md
index 964eed927c82..16d08cae30be 100644
--- a/gcc/config/riscv/autovec.md
+++ b/gcc/config/riscv/autovec.md
@@ -1396,7 +1396,7 @@
 
       /* Here we set VL = offset + 1.  */
       rtx length = gen_reg_rtx (Pmode);
-      operands[2] = gen_lowpart (Pmode, operands[2]);
+      operands[2] = convert_to_mode (Pmode, operands[2], true);
       if (CONST_INT_P (operands[2]))
 	  emit_move_insn (length, GEN_INT (INTVAL (operands[2]) + 1));
       else
@@ -1452,7 +1452,7 @@
 
     /* Emit the slide down to index 0 in a new vector.  */
     tmp = gen_reg_rtx (<MODE>mode);
-    operands[2] = gen_lowpart (Pmode, operands[2]);
+    operands[2] = convert_to_mode (Pmode, operands[2], true);
     rtx ops[] = {tmp, operands[1], operands[2]};
     riscv_vector::emit_vlmax_insn
       (code_for_pred_slide (UNSPEC_VSLIDEDOWN, <MODE>mode),
diff --git a/gcc/testsuite/gcc.target/riscv/pr126873.c b/gcc/testsuite/gcc.target/riscv/pr126873.c
new file mode 100644
index 000000000000..eab525d0cf20
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/pr126873.c
@@ -0,0 +1,37 @@
+/* { dg-do run { target rv64 } } */
+/* { dg-require-effective-target riscv_v } */
+/* { dg-require-effective-target rvv_zvl128b_ok } */
+/* { dg-options "-march=rv64gcv -mabi=lp64d -O1" } */
+
+typedef unsigned short u16 __attribute__ ((vector_size (4)));
+
+unsigned long long g;
+
+void __attribute__ ((noinline))
+f1 (unsigned long long a3)
+{
+  unsigned long long v15
+    = __builtin_bswap64 ((long long) 10398105857157080808ull
+			 / (long long) a3);
+  u16 bc13 = (u16) { 29637 };
+  if (0 >= bc13[(unsigned int) v15])
+    __builtin_abort ();
+}
+
+u16 __attribute__ ((noinline))
+f2 (u16 in)
+{
+  in[(unsigned int) g] = 123;
+  return in;
+}
+
+int
+main (void)
+{
+  f1 (17752357569705450221ull);
+  g = 0x0b00000000000000ull;
+  u16 r = f2 ((u16) { 1, 2 });
+  if (r[0] != 123 || r[1] != 2)
+    __builtin_abort ();
+  return 0;
+}