[PATCH v2] RISC-V: Make sure high bits of usadd operands is clean for HI/QI [PR116278]

[pan2 . li]

From: Pan Li <pan2...@intel.com>

For QI/HImode of .SAT_ADD,  the operands may be sign-extended and the
high bits of Xmode may be all 1 which is not expected.  For example as
below code.

signed char b[1];
unsigned short c;
signed char *d = b;
int main() {
  b[0] = -40;
  c = ({ (unsigned short)d[0] < 0xFFF6 ? (unsigned short)d[0] : 0xFFF6; }) + 9;
  __builtin_printf("%d\n", c);
}

After expanding we have:

;; _6 = .SAT_ADD (_3, 9);
(insn 8 7 9 (set (reg:DI 143)
        (high:DI (symbol_ref:DI ("d") [flags 0x86]  <var_decl d>)))
     (nil))
(insn 9 8 10 (set (reg/f:DI 142)
        (mem/f/c:DI (lo_sum:DI (reg:DI 143)
                (symbol_ref:DI ("d") [flags 0x86]  <var_decl d>)) [1 d+0 S8 
A64]))
     (nil))
(insn 10 9 11 (set (reg:HI 144 [ _3 ])
        (sign_extend:HI (mem:QI (reg/f:DI 142) [0 *d.0_1+0 S1 A8]))) 
"test.c":7:10 -1
     (nil))

The convert from signed char to unsigned short will have sign_extend rtl
as above.  And finally become the lb insn as below:

lb      a1,0(a5)   // a1 is -40, aka 0xffffffffffffffd8
lui     a0,0x1a
addi    a5,a1,9
slli    a5,a5,0x30
srli    a5,a5,0x30 // a5 is 65505
sltu    a1,a5,a1   // compare 65505 and 0xffffffffffffffd8 => TRUE

The sltu try to compare 65505 and 0xffffffffffffffd8 here,  but we
actually want to compare 65505 and 65496 (0xffd8).  Thus we need to
clean up the high bits to ensure this.

The below test suites are passed for this patch:
* The rv64gcv fully regression test.

        PR target/116278

gcc/ChangeLog:

        * config/riscv/riscv.cc (riscv_gen_zero_extend_rtx): Add new
        func impl to zero extend rtx.
        (riscv_expand_usadd): Leverage above func to cleanup operands
        and sum.

gcc/testsuite/ChangeLog:

        * gcc.target/riscv/pr116278-run-1.c: New test.

Signed-off-by: Pan Li <pan2...@intel.com>
---
 gcc/config/riscv/riscv.cc                     | 19 ++++++++++++++++++-
 .../gcc.target/riscv/pr116278-run-1.c         | 16 ++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.target/riscv/pr116278-run-1.c

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 5fe4273beb7..cfdb3d82972 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -11564,6 +11564,23 @@ riscv_get_raw_result_mode (int regno)
   return default_get_reg_raw_mode (regno);
 }
 
+/* Generate a new rtx of Xmode based on the rtx and mode in define pattern.
+   The rtx x will be zero extended to Xmode if the mode is HI/QImode,  and
+   the new zero extended Xmode rtx will be returned.
+   Or the gen_lowpart rtx of Xmode will be returned.  */
+
+static rtx
+riscv_gen_zero_extend_rtx (rtx x, machine_mode mode)
+{
+  if (mode != HImode && mode != QImode)
+    return gen_lowpart (Xmode, x);
+
+  rtx xmode_reg = gen_reg_rtx (Xmode);
+  riscv_emit_unary (ZERO_EXTEND, xmode_reg, x);
+
+  return xmode_reg;
+}
+
 /* Implements the unsigned saturation add standard name usadd for int mode.
 
    z = SAT_ADD(x, y).
@@ -11580,7 +11597,7 @@ riscv_expand_usadd (rtx dest, rtx x, rtx y)
   machine_mode mode = GET_MODE (dest);
   rtx xmode_sum = gen_reg_rtx (Xmode);
   rtx xmode_lt = gen_reg_rtx (Xmode);
-  rtx xmode_x = gen_lowpart (Xmode, x);
+  rtx xmode_x = riscv_gen_zero_extend_rtx (x, mode);
   rtx xmode_y = gen_lowpart (Xmode, y);
   rtx xmode_dest = gen_reg_rtx (Xmode);
 
diff --git a/gcc/testsuite/gcc.target/riscv/pr116278-run-1.c 
b/gcc/testsuite/gcc.target/riscv/pr116278-run-1.c
new file mode 100644
index 00000000000..f6268e290ec
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/pr116278-run-1.c
@@ -0,0 +1,16 @@
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+signed char b[1];
+int c;
+signed char *d = b;
+
+int main() {
+  b[0] = -40;
+  c = ({
+    (unsigned short)d[0] < 0xFFF6 ? (unsigned short)d[0] : 0xFFF6;
+  }) + 9;
+
+  if (c != 65505)
+    __builtin_abort ();
+}
-- 
2.43.0