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* [x86 PATCH] PR target/106577: force_reg may clobber operands during split.
@ 2022-08-12 20:40 Roger Sayle
  2022-08-15  7:45 ` Richard Biener
  0 siblings, 1 reply; 7+ messages in thread
From: Roger Sayle @ 2022-08-12 20:40 UTC (permalink / raw)
  To: gcc-patches

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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  <roger@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
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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);
+}

^ permalink raw reply	[flat|nested] 7+ messages in thread

end of thread, other threads:[~2022-12-02  9:51 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-08-12 20:40 [x86 PATCH] PR target/106577: force_reg may clobber operands during split Roger Sayle
2022-08-15  7:45 ` Richard Biener
2022-08-16  8:14   ` Richard Sandiford
2022-08-16  8:26     ` Richard Biener
2022-08-16  9:02       ` Richard Sandiford
2022-12-02  9:39     ` [PATCH] i386: Save/restore recog_data in ix86_vector_duplicate_value [PR106577] Jakub Jelinek
2022-12-02  9:51       ` Uros Bizjak

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