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Reapply r112176 without removing the other CMN patterns (that was unintentional).

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112206 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2010-08-26 18:33:51 +00:00
parent 446c6c41f5
commit 6165e87824
1 changed files with 46 additions and 2 deletions
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48
lib/Target/ARM/ARMInstrInfo.td
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@ -2297,8 +2297,52 @@ def : ARMV6Pat<(or (and GPR:$src1, 0xFFFF0000),
defm CMP : AI1_cmp_irs<0b1010, "cmp",
BinOpFrag<(ARMcmp node:$LHS, node:$RHS)>>;
//FIXME: Disable CMN, as CCodes are backwards from compare expectations
// Compare-to-zero still works out, just not the relationals
// FIXME: There seems to be a (potential) hardware bug with the CMN instruction
// and comparison with 0. These two pieces of code should give identical
// results:
//
// rsbs r1, r1, 0
// cmp r0, r1
// mov r0, #0
// it ls
// mov r0, #1
//
// and:
//
// cmn r0, r1
// mov r0, #0
// it ls
// mov r0, #1
//
// However, the CMN gives the *opposite* result when r1 is 0. This is because
// the carry flag is set in the CMP case but not in the CMN case. In short, the
// CMP instruction doesn't perform a truncate of the (logical) NOT of 0 plus the
// value of r0 and the carry bit (because the "carry bit" parameter to
// AddWithCarry is defined as 1 in this case, the carry flag will always be set
// when r0 >= 0). The CMN instruction doesn't perform a NOT of 0 so there is
// never a "carry" when this AddWithCarry is performed (because the "carry bit"
// parameter to AddWithCarry is defined as 0).
//
// The AddWithCarry in the CMP case seems to be relying upon the identity:
//
// ~x + 1 = -x
//
// However when x is 0 and unsigned, this doesn't hold:
//
// x = 0
// ~x = 0xFFFF FFFF
// ~x + 1 = 0x1 0000 0000
// (-x = 0) != (0x1 0000 0000 = ~x + 1)
//
// Therefore, we should disable *all* versions of CMN, especially when comparing
// against zero, until we can limit when the CMN instruction is used (when we
// know that the RHS is not 0) or when we have a hardware fix for this.
//
// (See the ARM docs for the "AddWithCarry" pseudo-code.)
//
// This is related to <rdar://problem/7569620>.
//
//defm CMN : AI1_cmp_irs<0b1011, "cmn",
// BinOpFrag<(ARMcmp node:$LHS,(ineg node:$RHS))>>;