ARM: Fix incorrect pack pattern

A "pkhtb x, x, y asr #num" uses the lower 16 bits of "y asr #num" and packs them
in the bottom half of "x". An arithmetic and logic shift are only equivalent in
this context if the shift amount is 16. We would be shifting in ones into the
bottom 16bits instead of zeros if "y" is negative.

radar://14338767

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185712 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 18 insertions, 3 deletions

diff --git a/lib/Target/ARM/ARMInstrInfo.td b/lib/Target/ARM/ARMInstrInfo.td
index ed68b4e..75d3de9 100644
--- a/lib/Target/ARM/ARMInstrInfo.td
+++ b/lib/Target/ARM/ARMInstrInfo.td
@@ -4011,9 +4011,11 @@ def PKHTB : APKHI<0b01101000, 1, (outs GPRnopc:$Rd),
 
 // Alternate cases for PKHTB where identities eliminate some nodes.  Note that
 // a shift amount of 0 is *not legal* here, it is PKHBT instead.
+// We also can not replace a srl (17..31) by an arithmetic shift we would use in
+// pkhtb src1, src2, asr (17..31).
 def : ARMV6Pat<(or (and GPRnopc:$src1, 0xFFFF0000),
-                   (srl GPRnopc:$src2, imm16_31:$sh)),
-               (PKHTB GPRnopc:$src1, GPRnopc:$src2, imm16_31:$sh)>;
+                   (srl GPRnopc:$src2, imm16:$sh)),
+               (PKHTB GPRnopc:$src1, GPRnopc:$src2, imm16:$sh)>;
 def : ARMV6Pat<(or (and GPRnopc:$src1, 0xFFFF0000),
                    (and (srl GPRnopc:$src2, imm1_15:$sh), 0xFFFF)),
                (PKHTB GPRnopc:$src1, GPRnopc:$src2, imm1_15:$sh)>;
diff --git a/test/CodeGen/ARM/pack.ll b/test/CodeGen/ARM/pack.ll
index 9015176..b944143 100644
--- a/test/CodeGen/ARM/pack.ll
+++ b/test/CodeGen/ARM/pack.ll
@@ -78,11 +78,24 @@ define i32 @test7(i32 %X, i32 %Y) {
 	ret i32 %tmp57
 }
 
+; Arithmetic and logic right shift does not have the same semantics if shifting
+; by more than 16 in this context.
+
 ; CHECK: test8
-; CHECK: pkhtb   r0, r0, r1, asr #22
+; CHECK-NOT: pkhtb   r0, r0, r1, asr #22
 define i32 @test8(i32 %X, i32 %Y) {
 	%tmp1 = and i32 %X, -65536
 	%tmp3 = lshr i32 %Y, 22
 	%tmp57 = or i32 %tmp3, %tmp1
 	ret i32 %tmp57
 }
+
+; CHECK: test9:
+; CHECK: pkhtb r0, r0, r1, asr #16
+define i32 @test9(i32 %src1, i32 %src2) {
+entry:
+    %tmp = and i32 %src1, -65536
+    %tmp2 = lshr i32 %src2, 16
+    %tmp3 = or i32 %tmp, %tmp2
+    ret i32 %tmp3
+}