From: Jakub Jelinek <jakub@redhat.com>
Date: Tue, 26 Jan 2021 13:48:26 +0000 (+0100)
Subject: aarch64: Tighten up checks for ubfix [PR98681]
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=17ad8cdebe65b47d257d85849747b806af0a85fd;p=gcc.git

aarch64: Tighten up checks for ubfix [PR98681]

The testcase in the patch doesn't assemble, because the instruction requires
that the penultimate operand (lsb) range is [0, 32] (or [0, 64]) and the last
operand's range is [1, 32 - lsb] (or [1, 64 - lsb]).
The INTVAL (shft_amnt) < GET_MODE_BITSIZE (mode) will accept the lsb operand
to be in range [MIN, 32] (or [MIN, 64]) and then we invoke UB in the
compiler and sometimes it will make it through.
The patch changes all the INTVAL uses in that function to UINTVAL,
which isn't strictly necessary, but can be done (e.g. after the
UINTVAL (shft_amnt) < GET_MODE_BITSIZE (mode) check we know it is not
negative and thus INTVAL (shft_amnt) and UINTVAL (shft_amnt) then behave the
same.  But, I had to add INTVAL (mask) > 0 check in that case, otherwise we
risk (hypothetically) emitting instruction that doesn't assemble.
The problem is with masks that have the MSB bit set, while the instruction
can handle those, e.g.
ubfiz w1, w0, 13, 19
will do
(w0 << 13) & 0xffffe000
in RTL we represent SImode constants with MSB set as negative HOST_WIDE_INT,
so it will actually be HOST_WIDE_INT_C (0xffffffffffffe000), and
the instruction uses %P3 to print the last operand, which calls
asm_fprintf (f, "%u", popcount_hwi (INTVAL (x)))
to print that.  But that will not print 19, but 51 instead, will include
there also all the copies of the sign bit.
Not supporting those masks with MSB set isn't a big loss though, they really
shouldn't appear normally, as both GIMPLE and RTL optimizations should
optimize those away (one isn't masking any bits off with such masks, so
just w0 << 13 will do too).

2021-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR target/98681
	* config/aarch64/aarch64.c (aarch64_mask_and_shift_for_ubfiz_p):
	Use UINTVAL (shft_amnt) and UINTVAL (mask) instead of INTVAL (shft_amnt)
	and INTVAL (mask).  Add && INTVAL (mask) > 0 condition.

	* gcc.c-torture/execute/pr98681.c: New test.
---

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index c19dc6cd895..b6192e55521 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -12060,10 +12060,11 @@ aarch64_mask_and_shift_for_ubfiz_p (scalar_int_mode mode, rtx mask,
 				    rtx shft_amnt)
 {
   return CONST_INT_P (mask) && CONST_INT_P (shft_amnt)
-	 && INTVAL (shft_amnt) < GET_MODE_BITSIZE (mode)
-	 && exact_log2 ((INTVAL (mask) >> INTVAL (shft_amnt)) + 1) >= 0
-	 && (INTVAL (mask)
-	     & ((HOST_WIDE_INT_1U << INTVAL (shft_amnt)) - 1)) == 0;
+	 && INTVAL (mask) > 0
+	 && UINTVAL (shft_amnt) < GET_MODE_BITSIZE (mode)
+	 && exact_log2 ((UINTVAL (mask) >> UINTVAL (shft_amnt)) + 1) >= 0
+	 && (UINTVAL (mask)
+	     & ((HOST_WIDE_INT_1U << UINTVAL (shft_amnt)) - 1)) == 0;
 }
 
 /* Return true if the masks and a shift amount from an RTX of the form
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr98681.c b/gcc/testsuite/gcc.c-torture/execute/pr98681.c
new file mode 100644
index 00000000000..a256881342f
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr98681.c
@@ -0,0 +1,18 @@
+/* PR target/98681 */
+
+__attribute__((noipa)) int
+foo (int x)
+{
+  if (x > 32)
+    return (x << -64) & 255;
+  else
+    return x;
+}
+
+int
+main ()
+{
+  if (foo (32) != 32 || foo (-150) != -150)
+    __builtin_abort ();
+  return 0;
+}