Fix ARM handling of tBcc branch relaxation.

rdar://10069056

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145885 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 19 insertions, 3 deletions

diff --git a/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp b/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
index 59fae27..502a48b 100644
--- a/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
+++ b/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
@@ -146,11 +146,13 @@ bool ARMAsmBackend::fixupNeedsRelaxation(const MCFixup &Fixup,
                                          uint64_t Value,
                                          const MCInstFragment *DF,
                                          const MCAsmLayout &Layout) const {
-  // FIXME:  This isn't correct for ARM. Just moving the "generic" logic
-  // into the targets for now.
+  // Relaxing tBcc to t2Bcc. tBcc has a signed 9-bit displacement with the
+  // low bit being an implied zero. There's an implied +4 offset for the
+  // branch, so we adjust the other way here to determine what's
+  // encodable.
   //
   // Relax if the value is too big for a (signed) i8.
-  return int64_t(Value) != int64_t(int8_t(Value));
+  return int64_t((Value - 4)>>1) != int64_t(int8_t((Value - 4)>>1));
 }
 
 void ARMAsmBackend::RelaxInstruction(const MCInst &Inst, MCInst &Res) const {
diff --git a/test/MC/MachO/relax-thumb2-branches.s b/test/MC/MachO/relax-thumb2-branches.s
new file mode 100644
index 0000000..7916d42
--- /dev/null
+++ b/test/MC/MachO/relax-thumb2-branches.s
@@ -0,0 +1,14 @@
+@ RUN: llvm-mc -triple=thumbv7-apple-darwin -show-encoding %s -filetype=obj -o - | macho-dump --dump-section-data | FileCheck %s
+
+        ble Lfoo        @ wide encoding
+
+        .space 258
+Lfoo:
+        nop
+
+        ble Lbaz        @ narrow encoding
+        .space 256
+Lbaz:
+
+@ CHECK: '_section_data', '40f38180
+@ CHECK: 000000bf 7fdd