[ARM] ldr pc,=expression should be allowed in Thumb2

[peter.smith]

Summary

This change allows the PC to be used as a destination register for the pseudo instruction LDR Rt,=expression . The pseudo instruction must not be transformed into a MOV, but it can use the Thumb2 LDR (literal) instruction to a constant pool entry. See (A7.7.43 from ARMv7M ARM ARM).

There are no changes required to ARMAsmParser.cpp as the use of PC or SP already inhibits the transformation to MOV.

Link to v7 ARM ARM that does not require registration: https://developer.arm.com/products/architecture/m-profile/docs/ddi0403/e/armv7-m-architecture-reference-manual

[olista01]

LGTM.

llvm/trunk/lib/Target/ARM/ARMInstrThumb2.td

def : t2InstAlias<"ldrsh${p}.w $Rt, $addr",
                   (t2LDRSHpcrel GPRnopc:$Rt, t2ldr_pcrel_imm12:$addr, pred:$p)>;
 
 def : t2InstAlias<"add${p} $Rd, pc, $imm",
                   (t2ADR rGPR:$Rd, imm0_4095:$imm, pred:$p)>;
 
 // Pseudo instruction ldr Rt, =immediate
 def t2LDRConstPool
   : t2AsmPseudo<"ldr${p} $Rt, $immediate",
-                (ins GPRnopc:$Rt, const_pool_asm_imm:$immediate, pred:$p)>;
+                (ins GPR:$Rt, const_pool_asm_imm:$immediate, pred:$p)>;
 // Version w/ the .w suffix.
 def : t2InstAlias<"ldr${p}.w $Rt, $immediate",
                   (t2LDRConstPool GPRnopc:$Rt,
                   const_pool_asm_imm:$immediate, pred:$p)>;
 
 // PLD/PLDW/PLI with alternate literal form.
 def : t2InstAlias<"pld${p} $addr",
                   (t2PLDpci t2ldr_pcrel_imm12:$addr, pred:$p)>;
 def : InstAlias<"pli${p} $addr",
                  (t2PLIpci  t2ldr_pcrel_imm12:$addr, pred:$p), 0>,
       Requires<[IsThumb2,HasV7]>;

llvm/trunk/test/MC/ARM/ldr-pseudo-unpredictable.s

 @RUN: llvm-mc -triple armv5-unknown-linux-gnueabi %s | FileCheck --check-prefix=CHECK-ARM %s
-@RUN: not llvm-mc -triple thumbv7-unknown-linux-gnueabi %s 2>&1 | FileCheck --check-prefix=CHECK-SP %s
+@RUN: llvm-mc -triple thumbv7-unknown-linux-gnueabi %s 2>&1 | FileCheck --check-prefix=CHECK-T2 %s
 @RUN: not llvm-mc -triple thumbv5-unknown-linux-gnueabi %s 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
 @RUN: llvm-mc -triple armv5-base-apple-darwin %s | FileCheck --check-prefix=CHECK-DARWIN-ARM %s
-@RUN: not llvm-mc -triple thumbv7-base-apple-darwin %s 2>&1 | FileCheck --check-prefix=CHECK-DARWIN-SP %s
+@RUN: llvm-mc -triple thumbv7-base-apple-darwin %s 2>&1 | FileCheck --check-prefix=CHECK-DARWIN-T2 %s
 @RUN: not llvm-mc -triple thumbv5-base.apple.darwin %s 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
 
 @ We dont't do the transformation for rt = sp or pc
 @ as it is unpredictable for many of the MOV encondings
   ldr pc, = 0x4
 @ CHECK-ARM: ldr pc, .Ltmp[[TMP0:[0-9]+]]
 @ CHECK-DARWIN-ARM: ldr pc, Ltmp0
-@ CHECK-SP: error: instruction requires: arm-mode
-@ CHECK-DARWIN-SP: error: instruction requires: arm-mode
-@ CHECK-NONE: error: instruction requires: arm-mode
+@ CHECK-T2: ldr.w pc, .Ltmp[[TMP0:[0-9]+]]
+@ CHECK-DARWIN-T2: ldr.w pc, Ltmp0
+@ CHECK-NONE: error: instruction requires: thumb2
   ldr sp, = 0x8
 @ CHECK-ARM: ldr sp, .Ltmp[[TMP1:[0-9]+]]
 @ CHECK-DARWIN-ARM: ldr sp, Ltmp1
-@ CHECK-SP: ldr.w sp, .Ltmp[[TMP0:[0-9]+]]
-@ CHECK-DARWIN-SP: ldr.w sp, Ltmp0
-@ CHECK-NONE: error: instruction requires: arm-mode
+@ CHECK-T2: ldr.w sp, .Ltmp[[TMP1:[0-9]+]]
+@ CHECK-DARWIN-T2: ldr.w sp, Ltmp1
+@ CHECK-NONE: error: instruction requires: thumb2