[x86 PATCH] PR target/106577: force_reg may clobber operands during split.

[Roger Sayle]

This patch fixes PR target/106577 which is a recent ICE on valid regression
caused by my introduction of a *testti_doubleword pre-reload splitter in
i386.md.  During the split pass before reload, this converts the virtual
*testti_doubleword into an *andti3_doubleword and *cmpti_doubleword,
checking that any immediate operand is a valid "x86_64_hilo_general_operand"
and placing it into a TImode register using force_reg if it isn't.

The unexpected behaviour (that caught me out) is that calling force_reg
may occasionally clobber the contents of the global operands array, or
more accurately recog_data.operand[0], which means that by the time
split_XXX calls gen_split_YYY the replacement insn's operands have been
corrupted.

It's difficult to tell who (if anyone is at fault).  The re-entrant
stack trace (for the attached PR) looks like:

gen_split_203 (*testti_doubleword) calls
force_reg calls
emit_move_insn calls
emit_move_insn_1 calls
gen_movti calls
ix86_expand_move calls
ix86_convert_const_wide_int_to_broadcast calls
ix86_vector_duplicate_value calls
recog_memoized calls
recog.

By far the simplest and possibly correct fix is rather than attempt
to push and pop recog_data, to simply (in pre-reload splits) save a
copy of any operands that will be needed after force_reg, and use
these copies afterwards.  Many pre-reload splitters avoid this issue
using "[(clobber (const_int 0))]" and so avoid gen_split_YYY functions,
but in our case we still need to save a copy of operands[0] (even if we
call emit_insn or expand_* ourselves), so we might as well continue to
use the conveniently generated gen_split.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32},
with no new failures. Ok for mainline?


2022-08-12  Roger Sayle  <ro...@nextmovesoftware.com>

gcc/ChangeLog
        PR target/106577
        * config/i386/i386.md (*testti_doubleword): Preserve a copy of
        operands[0], and move initialization of operands[2] later, as the
        call to force_reg may clobber the contents of the operands array.

gcc/testsuite/ChangeLog
        PR target/106577
        * gcc.target/i386/pr106577.c: New test case.


Thanks,
Roger
--


diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 2fde8cd..e9232cd 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -9772,9 +9772,12 @@
               (clobber (reg:CC FLAGS_REG))])
    (set (reg:CCZ FLAGS_REG) (compare:CCZ (match_dup 2) (const_int 0)))]
 {
-  operands[2] = gen_reg_rtx (TImode);
+  /* Calling force_reg may clobber operands[0].  */
+  rtx save_op0 = operands[0];
   if (!x86_64_hilo_general_operand (operands[1], TImode))
     operands[1] = force_reg (TImode, operands[1]);
+  operands[2] = gen_reg_rtx (TImode);
+  operands[0] = save_op0;
 })
 
 ;; Combine likes to form bit extractions for some tests.  Humor it.
diff --git a/gcc/testsuite/gcc.target/i386/pr106577.c 
b/gcc/testsuite/gcc.target/i386/pr106577.c
new file mode 100644
index 0000000..4e35031
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr106577.c
@@ -0,0 +1,7 @@
+/* { dg-do compile { target int128 } } */
+/* { dg-options "-O2 -mavx" } */
+
+int i;
+void foo(void) {
+  i ^= !(((unsigned __int128)0xf0f0f0f0f0f0f0f0 << 64 | 0xf0f0f0f0f0f0f0f0) & 
i);
+}