It's incorrect to use CMN to compare with a negated operand if the
following condition is an inequality. This is because of boundary
conditions when the negated operations overflow (or when zero), since
the flags are then not the swapped version of the comparison.
This patch restricts the pattern to CC_Zmode (a new mode for the
comparison flags).
gcc:
PR target/59744
* aarch64-modes.def (CC_Zmode): New flags mode.
* aarch64.c (aarch64_select_cc_mode): Only allow NEG when the
condition represents an equality.
(aarch64_get_condition_code0): Handle CC_Zmode.
* aarch64.md (compare_neg<mode>): Restrict to equality
operations.
testsuite:
PR target/59744
* gcc.target/aarch64/cmn-neg.c: Use equality comparisons.
* gcc.target/aarch64/cmn-neg2.c: New test.
Committed to trunk.
R.--- gcc/config/aarch64/aarch64-modes.def (revision 206483)
+++ gcc/config/aarch64/aarch64-modes.def (local)
@@ -24,6 +24,7 @@ CC_MODE (CC_SWP);
CC_MODE (CC_ZESWP); /* zero-extend LHS (but swap to make it RHS). */
CC_MODE (CC_SESWP); /* sign-extend LHS (but swap to make it RHS). */
CC_MODE (CC_NZ); /* Only N and Z bits of condition flags are valid. */
+CC_MODE (CC_Z); /* Only Z bit of condition flags is valid. */
/* Vector modes. */
VECTOR_MODES (INT, 8); /* V8QI V4HI V2SI. */
--- gcc/config/aarch64/aarch64.c (revision 206483)
+++ gcc/config/aarch64/aarch64.c (local)
@@ -3326,17 +3326,24 @@ aarch64_select_cc_mode (RTX_CODE code, r
|| GET_CODE (x) == NEG))
return CC_NZmode;
- /* A compare with a shifted or negated operand. Because of canonicalization,
+ /* A compare with a shifted operand. Because of canonicalization,
the comparison will have to be swapped when we emit the assembly
code. */
if ((GET_MODE (x) == SImode || GET_MODE (x) == DImode)
&& (GET_CODE (y) == REG || GET_CODE (y) == SUBREG)
&& (GET_CODE (x) == ASHIFT || GET_CODE (x) == ASHIFTRT
|| GET_CODE (x) == LSHIFTRT
- || GET_CODE (x) == ZERO_EXTEND || GET_CODE (x) == SIGN_EXTEND
- || GET_CODE (x) == NEG))
+ || GET_CODE (x) == ZERO_EXTEND || GET_CODE (x) == SIGN_EXTEND))
return CC_SWPmode;
+ /* Similarly for a negated operand, but we can only do this for
+ equalities. */
+ if ((GET_MODE (x) == SImode || GET_MODE (x) == DImode)
+ && (GET_CODE (y) == REG || GET_CODE (y) == SUBREG)
+ && (code == EQ || code == NE)
+ && GET_CODE (x) == NEG)
+ return CC_Zmode;
+
/* A compare of a mode narrower than SI mode against zero can be done
by extending the value in the comparison. */
if ((GET_MODE (x) == QImode || GET_MODE (x) == HImode)
@@ -3427,6 +3434,15 @@ aarch64_get_condition_code (rtx x)
}
break;
+ case CC_Zmode:
+ switch (comp_code)
+ {
+ case NE: return AARCH64_NE;
+ case EQ: return AARCH64_EQ;
+ default: gcc_unreachable ();
+ }
+ break;
+
default:
gcc_unreachable ();
break;
--- gcc/config/aarch64/aarch64.md (revision 206483)
+++ gcc/config/aarch64/aarch64.md (local)
@@ -1250,8 +1250,8 @@ (define_insn "*add<mode>3nr_compare0"
)
(define_insn "*compare_neg<mode>"
- [(set (reg:CC_SWP CC_REGNUM)
- (compare:CC_SWP
+ [(set (reg:CC_Z CC_REGNUM)
+ (compare:CC_Z
(neg:GPI (match_operand:GPI 0 "register_operand" "r"))
(match_operand:GPI 1 "register_operand" "r")))]
""
--- gcc/testsuite/gcc.target/aarch64/cmn-neg.c (revision 206483)
+++ gcc/testsuite/gcc.target/aarch64/cmn-neg.c (local)
@@ -6,7 +6,7 @@ extern void abort (void);
void __attribute__ ((noinline))
foo_s32 (int a, int b)
{
- if (a < -b)
+ if (a == -b)
abort ();
}
/* { dg-final { scan-assembler "cmn\tw\[0-9\]" } } */
@@ -14,7 +14,7 @@ foo_s32 (int a, int b)
void __attribute__ ((noinline))
foo_s64 (long long a, long long b)
{
- if (a < -b)
+ if (a == -b)
abort ();
}
/* { dg-final { scan-assembler "cmn\tx\[0-9\]" } } */
--- gcc/testsuite/gcc.target/aarch64/cmn-neg2.c (revision 0)
+++ gcc/testsuite/gcc.target/aarch64/cmn-neg2.c (local)
@@ -0,0 +1,34 @@
+/* { dg-do run } */
+/* { dg-options "-O2 --save-temps" } */
+
+extern void abort (void);
+
+/* It's unsafe to use CMN in these comparisons. */
+
+void __attribute__ ((noinline))
+foo_s32 (int a, int b)
+{
+ if (a < -b)
+ abort ();
+}
+
+void __attribute__ ((noinline))
+foo_s64 (unsigned long long a, unsigned long long b)
+{
+ if (a > -b)
+ abort ();
+}
+
+
+int
+main (void)
+{
+ int a = 30;
+ int b = 42;
+ foo_s32 (a, b);
+ foo_s64 (a, b);
+ return 0;
+}
+/* { dg-final { scan-assembler-not "cmn\t" } } */
+
+/* { dg-final { cleanup-saved-temps } } */
