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Reword since this may not be a bug but intended behavior.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113584 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2010-09-10 10:31:11 +00:00
parent a64eb8357e
commit c8714bb144
1 changed files with 6 additions and 10 deletions
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16
lib/Target/ARM/ARMInstrInfo.td
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@ -2310,8 +2310,8 @@ def : ARMV6Pat<(or (and GPR:$src1, 0xFFFF0000),
defm CMP : AI1_cmp_irs<0b1010, "cmp",
BinOpFrag<(ARMcmp node:$LHS, node:$RHS)>>;
// 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
// FIXME: We have to be careful when using the CMN instruction and comparison
// with 0. One would expect these two pieces of code should give identical
// results:
//
// rsbs r1, r1, 0
@ -2336,20 +2336,16 @@ defm CMP : AI1_cmp_irs<0b1010, "cmp",
// 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:
// When x is 0 and unsigned:
//
// 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.
// Therefore, we should disable CMN 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 it's a comparison which doesn't look at the 'carry' flag).
//
// (See the ARM docs for the "AddWithCarry" pseudo-code.)
//