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| author | Bill Wendling <isanbard@gmail.com> | 2010-08-26 18:33:51 +0000 |
|---|---|---|
| committer | Bill Wendling <isanbard@gmail.com> | 2010-08-26 18:33:51 +0000 |
| commit | 877732be562940cfe7532f26f27691473ab210e4 (patch) | |
| tree | 5c1cf4a73e32dc1a9cbee064216db4973abf8529 | |
| parent | 0aecebd8445cb84cb763d069f909d59ee683f8a8 (diff) | |
| download | external_llvm-877732be562940cfe7532f26f27691473ab210e4.zip external_llvm-877732be562940cfe7532f26f27691473ab210e4.tar.gz external_llvm-877732be562940cfe7532f26f27691473ab210e4.tar.bz2 | |
Reapply r112176 without removing the other CMN patterns (that was unintentional).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112206 91177308-0d34-0410-b5e6-96231b3b80d8
| -rw-r--r-- | lib/Target/ARM/ARMInstrInfo.td | 48 |
1 files changed, 46 insertions, 2 deletions
diff --git a/lib/Target/ARM/ARMInstrInfo.td b/lib/Target/ARM/ARMInstrInfo.td index 298e6d3..15206a1 100644 --- a/lib/Target/ARM/ARMInstrInfo.td +++ b/lib/Target/ARM/ARMInstrInfo.td @@ -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))>>; |