This fixes one divergence between LLVM and binutils for ARM in the

text section.

Assume the following bit of annotated assembly:

.section	.data.rel.ro,"aw",%progbits
.align	2
.LAlpha:
.long	startval(GOTOFF)

.text
.align	2
.type	main,%function
.align	4

main: ;;; assume "main" starts at offset 0x20
0x0	push	{r11, lr}
0x4	movw	r0, :lower16:(.LAlpha-(.LBeta+8))
;;; ==> (.AddrOf(.LAlpha) - ((.AddrOf(.LBeta) - .AddrOf(".")) + 8)
;;; ==> (??? - ((16-4) + 8) = -20
0x8	movt	r0, :upper16:(.LAlpha-(.LBeta+8))
;;; ==> (.AddrOf(.LAlpha) - ((.AddrOf(.LBeta) - .AddrOf(".")) + 8)
;;; ==> (??? - ((16-8) + 8) = -16
0xc	... blah

.LBeta:
0x10	add	r0, pc, r0
0x14	... blah

.LGamma:
0x18	add	r1, pc, r1

Above snippet results in the following relocs in the .o file for the
first pair of movw/movt instructions

00000024 R_ARM_MOVW_PREL_NC .LAlpha
00000028 R_ARM_MOVT_PREL .LAlpha

And the encoded instructions in the .o file for main: must be

00000020 <main>:
20:	e92d4800 push	{fp, lr}
24:	e30f0fec movw	r0, #65516	; 0xffec i.e. -20
28:	e34f0ff0 movt	r0, #65520	; 0xfff0 i.e. -16

However, llc (prior to this commit) generates the following sequence

00000020 <main>:
20:	e92d4800 push	{fp, lr}
24:	e30f0fec movw	r0, #65516	; 0xffec - i.e. -20
28:	e34f0fff movt	r0, #65535	; 0xffff - i.e. -1

What has to happen in the ArmAsmBackend is that if the relocation is PC
relative, the 16 bits encoded as part of movw and movt must be both addends,
not addresses. It makes sense to encode addresses by right shifting the value
by 16, but the result is incorrect for PIC.
i.e., the right shift by 16 for movt is ONLY valid for the NON-PCRel case.

This change agrees with what GNU as does, and makes the PIC code run.

MC/ARM/elf-movt.s covers this case.

llvm-svn: 131674

1 files changed, 6 insertions, 3 deletions

diff --git a/llvm/lib/Target/ARM/ARMAsmBackend.cpp b/llvm/lib/Target/ARM/ARMAsmBackend.cpp
index d6a170a7065..2cfd703d283 100644
--- a/llvm/lib/Target/ARM/ARMAsmBackend.cpp
+++ b/llvm/lib/Target/ARM/ARMAsmBackend.cpp
@@ -164,23 +164,25 @@ static unsigned adjustFixupValue(unsigned Kind, uint64_t Value) {
   case FK_Data_4:
     return Value;
   case ARM::fixup_arm_movt_hi16:
-  case ARM::fixup_arm_movt_hi16_pcrel:
     Value >>= 16;
     // Fallthrough
   case ARM::fixup_arm_movw_lo16:
+  case ARM::fixup_arm_movt_hi16_pcrel:
   case ARM::fixup_arm_movw_lo16_pcrel: {
     unsigned Hi4 = (Value & 0xF000) >> 12;
     unsigned Lo12 = Value & 0x0FFF;
+    assert ((((int64_t)Value) >= -0x8000) && (((int64_t)Value) <= 0x7fff) &&
+            "Out of range pc-relative fixup value!");
     // inst{19-16} = Hi4;
     // inst{11-0} = Lo12;
     Value = (Hi4 << 16) | (Lo12);
     return Value;
   }
   case ARM::fixup_t2_movt_hi16:
-  case ARM::fixup_t2_movt_hi16_pcrel:
     Value >>= 16;
     // Fallthrough
   case ARM::fixup_t2_movw_lo16:
+  case ARM::fixup_t2_movt_hi16_pcrel:
   case ARM::fixup_t2_movw_lo16_pcrel: {
     unsigned Hi4 = (Value & 0xF000) >> 12;
     unsigned i = (Value & 0x800) >> 11;
@@ -190,8 +192,9 @@ static unsigned adjustFixupValue(unsigned Kind, uint64_t Value) {
     // inst{26} = i;
     // inst{14-12} = Mid3;
     // inst{7-0} = Lo8;
+    assert ((((int64_t)Value) >= -0x8000) && (((int64_t)Value) <= 0x7fff) &&
+            "Out of range pc-relative fixup value!");
     Value = (Hi4 << 16) | (i << 26) | (Mid3 << 12) | (Lo8);
-
     uint64_t swapped = (Value & 0xFFFF0000) >> 16;
     swapped |= (Value & 0x0000FFFF) << 16;
     return swapped;