[Thumb] Teach ISel how to lower compares of AND bitmasks efficiently

This is essentially a recommit of r285893, but with a correctness fix. The
problem of the original commit was that this:

bic r5, r7, #31
cbz r5, .LBB2_10

got rewritten into:

lsrs  r5, r7, #5
beq .LBB2_10

The result in destination register r5 is not the same and this is incorrect
when r5 is not dead. So this fix includes checking the uses of the AND
destination register. And also, compared to the original commit, some regression
tests didn't need changing anymore because of this extra check.

For completeness, this was the original commit message:

For the common pattern (CMPZ (AND x, #bitmask), #0), we can do some more
efficient instruction selection if the bitmask is one consecutive sequence of
set bits (32 - clz(bm) - ctz(bm) == popcount(bm)).

1) If the bitmask touches the LSB, then we can remove all the upper bits and
set the flags by doing one LSLS.
2) If the bitmask touches the MSB, then we can remove all the lower bits and
set the flags with one LSRS.
3) If the bitmask has popcount == 1 (only one set bit), we can shift that bit
into the sign bit with one LSLS and change the condition query from NE/EQ to
MI/PL (we could also implement this by shifting into the carry bit and
branching on BCC/BCS).
4) Otherwise, we can emit a sequence of LSLS+LSRS to remove the upper and lower
zero bits of the mask.

1-3 require only one 16-bit instruction and can elide the CMP. 4 requires two
16-bit instructions but can elide the CMP and doesn't require materializing a
complex immediate, so is also a win.

Differential Revision: https://reviews.llvm.org/D27761

llvm-svn: 289794

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/test/CodeGen/ARM/call-tc.ll b/llvm/test/CodeGen/ARM/call-tc.ll
index 2277a585336..c5cfb9def33 100644
--- a/llvm/test/CodeGen/ARM/call-tc.ll
+++ b/llvm/test/CodeGen/ARM/call-tc.ll
@@ -120,7 +120,7 @@ if.end:                                           ; preds = %entry
   br i1 %tobool2, label %if.end5, label %if.then3
 
 if.then3:                                         ; preds = %if.end
-; CHECKT2D: bne.w _b
+; CHECKT2D: bmi.w _b
   %call4 = tail call i32 @b(i32 %x) nounwind
   br label %return