[Thumb1] Resolve FIXME: Use 'mov hi, $src; mov $dst, hi'

Consider the following:

        ldr     r0, [r4]
        ldr     r7, [r0, #4]
        cmp     r7, r3
        bhi     .LBB0_6
        cmp     r0, r2
        push    {r0}
        pop     {r4}
        bne     .LBB0_3
        movs    r0, r6
        pop     {r4, r5, r6, r7}
        pop     {r1}
        bx      r1

Here is a snippet of the generated THUMB1 code of the K&R malloc function that clang currently compiles to.

push    {r0} ends up being popped to pop {r4}.

movs r4, r0 would destroy the flags set by cmp right above.

The compiler has no alternative in this case, except one:
the only alternative is to transfer through a high register.

However, it seems like LLVM does not consider that this is a valid approach, even though it is a free clobbering a high register.

This patch addresses the FIXME so the compiler can do that when it can in r10 or r11, or r12.

llvm/lib/Target/ARM/Thumb1InstrInfo.cpp

@@ -12,6 +12,8 @@
 
 #include "Thumb1InstrInfo.h"
 #include "ARMSubtarget.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
@@ -47,24 +49,57 @@ void Thumb1InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
   assert(ARM::GPRRegClass.contains(DestReg, SrcReg) &&
          "Thumb1 can only copy GPR registers");
 
-  if (st.hasV6Ops() || ARM::hGPRRegClass.contains(SrcReg)
-      || !ARM::tGPRRegClass.contains(DestReg))
+  if (st.hasV6Ops() || ARM::hGPRRegClass.contains(SrcReg) ||
+      !ARM::tGPRRegClass.contains(DestReg))
     BuildMI(MBB, I, DL, get(ARM::tMOVr), DestReg)
         .addReg(SrcReg, getKillRegState(KillSrc))
         .add(predOps(ARMCC::AL));
   else {
-    // FIXME: Can also use 'mov hi, $src; mov $dst, hi',
-    // with hi as either r10 or r11.
-
     const TargetRegisterInfo *RegInfo = st.getRegisterInfo();
-    if (MBB.computeRegisterLiveness(RegInfo, ARM::CPSR, I)
-        == MachineBasicBlock::LQR_Dead) {
+    LiveRegUnits UsedRegs(*RegInfo);
+    UsedRegs.addLiveOuts(MBB);
+
+    auto InstUpToI = MBB.end();
+    while (InstUpToI != I)
+      // The pre-decrement is on purpose here.
+      // We want to have the liveness right before I.
+      UsedRegs.stepBackward(*--InstUpToI);
+
+    if (UsedRegs.available(ARM::CPSR)) {
       BuildMI(MBB, I, DL, get(ARM::tMOVSr), DestReg)
           .addReg(SrcReg, getKillRegState(KillSrc))
           ->addRegisterDead(ARM::CPSR, RegInfo);
       return;
     }
 
+    // Use high register to move source to destination
+    // if movs is not an option.
+    BitVector Allocatable = RegInfo->getAllocatableSet(
+        MF, RegInfo->getRegClass(ARM::hGPRRegClassID));
+
+    Register TmpReg = ARM::NoRegister;
+    // Prefer R12 as it is known to not be preserved anyway
+    if (UsedRegs.available(ARM::R12) && Allocatable.test(ARM::R12)) {
+      TmpReg = ARM::R12;
+    } else {
+      for (Register Reg : Allocatable.set_bits()) {
+        if (UsedRegs.available(Reg)) {
+          TmpReg = Reg;
+          break;
+        }
+      }
+    }
+
+    if (TmpReg) {
+      BuildMI(MBB, I, DL, get(ARM::tMOVr), TmpReg)
+          .addReg(SrcReg, getKillRegState(KillSrc))
+          .add(predOps(ARMCC::AL));
+      BuildMI(MBB, I, DL, get(ARM::tMOVr), DestReg)
+          .addReg(TmpReg, getKillRegState(true))
+          .add(predOps(ARMCC::AL));
+      return;
+    }
+
     // 'MOV lo, lo' is unpredictable on < v6, so use the stack to do it
     BuildMI(MBB, I, DL, get(ARM::tPUSH))
         .add(predOps(ARMCC::AL))

llvm/test/CodeGen/ARM/sadd_sat.ll

@@ -130,8 +130,8 @@ define i64 @func2(i64 %x, i64 %y) nounwind {
 ; CHECK-T15TE-NEXT:    bics r4, r1
 ; CHECK-T15TE-NEXT:    asrs r1, r3, #31
 ; CHECK-T15TE-NEXT:    cmp r4, #0
-; CHECK-T15TE-NEXT:    push {r1}
-; CHECK-T15TE-NEXT:    pop {r0}
+; CHECK-T15TE-NEXT:    mov r12, r1
+; CHECK-T15TE-NEXT:    mov r0, r12
 ; CHECK-T15TE-NEXT:    bmi .LBB1_2
 ; CHECK-T15TE-NEXT:  @ %bb.1:
 ; CHECK-T15TE-NEXT:    movs r0, r2

llvm/test/CodeGen/ARM/select_const.ll

@@ -665,8 +665,8 @@ define i64 @opaque_constant1(i1 %cond, i64 %x) {
 ; THUMB-NEXT:    movs r7, #1
 ; THUMB-NEXT:    ands r0, r7
 ; THUMB-NEXT:    subs r1, r0, #1
-; THUMB-NEXT:    push {r0}
-; THUMB-NEXT:    pop {r4}
+; THUMB-NEXT:    mov r12, r0
+; THUMB-NEXT:    mov r4, r12
 ; THUMB-NEXT:    sbcs r4, r1
 ; THUMB-NEXT:    cmp r0, #0
 ; THUMB-NEXT:    bne .LBB24_2
@@ -681,8 +681,8 @@ define i64 @opaque_constant1(i1 %cond, i64 %x) {
 ; THUMB-NEXT:    ands r5, r0
 ; THUMB-NEXT:    movs r6, #0
 ; THUMB-NEXT:    subs r0, r5, #1
-; THUMB-NEXT:    push {r4}
-; THUMB-NEXT:    pop {r1}
+; THUMB-NEXT:    mov r12, r4
+; THUMB-NEXT:    mov r1, r12
 ; THUMB-NEXT:    sbcs r1, r6
 ; THUMB-NEXT:    eors r3, r7
 ; THUMB-NEXT:    ldr r6, .LCPI24_0
@@ -786,11 +786,11 @@ define i64 @func(i64 %arg) {
 ; THUMB-NEXT:    push {r4, lr}
 ; THUMB-NEXT:    movs r2, #0
 ; THUMB-NEXT:    adds r3, r0, #1
-; THUMB-NEXT:    push {r1}
-; THUMB-NEXT:    pop {r3}
+; THUMB-NEXT:    mov r12, r1
+; THUMB-NEXT:    mov r3, r12
 ; THUMB-NEXT:    adcs r3, r2
-; THUMB-NEXT:    push {r2}
-; THUMB-NEXT:    pop {r3}
+; THUMB-NEXT:    mov r12, r2
+; THUMB-NEXT:    mov r3, r12
 ; THUMB-NEXT:    adcs r3, r2
 ; THUMB-NEXT:    subs r4, r3, #1
 ; THUMB-NEXT:    adds r0, r0, #1

llvm/test/CodeGen/ARM/wide-compares.ll

@@ -257,12 +257,12 @@ define {i32, i32} @test_slt_not(i32 %c, i32 %d, i64 %a, i64 %b) {
 ; CHECK-THUMB1-NOMOV-NEXT:    ldr r5, [sp, #16]
 ; CHECK-THUMB1-NOMOV-NEXT:    subs r2, r2, r5
 ; CHECK-THUMB1-NOMOV-NEXT:    sbcs r3, r0
-; CHECK-THUMB1-NOMOV-NEXT:    push {r1}
-; CHECK-THUMB1-NOMOV-NEXT:    pop {r0}
+; CHECK-THUMB1-NOMOV-NEXT:    mov r12, r1
+; CHECK-THUMB1-NOMOV-NEXT:    mov r0, r12
 ; CHECK-THUMB1-NOMOV-NEXT:    blt .LBB3_2
 ; CHECK-THUMB1-NOMOV-NEXT:  @ %bb.1: @ %entry
-; CHECK-THUMB1-NOMOV-NEXT:    push {r4}
-; CHECK-THUMB1-NOMOV-NEXT:    pop {r0}
+; CHECK-THUMB1-NOMOV-NEXT:    mov r12, r4
+; CHECK-THUMB1-NOMOV-NEXT:    mov r0, r12
 ; CHECK-THUMB1-NOMOV-NEXT:  .LBB3_2: @ %entry
 ; CHECK-THUMB1-NOMOV-NEXT:    bge .LBB3_4
 ; CHECK-THUMB1-NOMOV-NEXT:  @ %bb.3: @ %entry

llvm/test/CodeGen/Thumb/pr35836.ll

@@ -35,18 +35,18 @@ while.body:
   br label %while.body
 }
 ; CHECK: adds	r3, r0, r1
-; CHECK: push	{r5}
-; CHECK: pop	{r1}
+; CHECK: mov	r12, r5
+; CHECK: mov	r1, r12
 ; CHECK: adcs	r1, r5
 ; CHECK: ldr	r0, [sp, #12]           @ 4-byte Reload
 ; CHECK: ldr	r2, [sp, #8]            @ 4-byte Reload
 ; CHECK: adds	r2, r0, r2
-; CHECK: push	{r5}
-; CHECK: pop	{r4}
+; CHECK: mov	r12, r5
+; CHECK: mov	r4, r12
 ; CHECK: adcs	r4, r5
 ; CHECK: adds	r0, r2, r5
-; CHECK: push	{r3}
-; CHECK: pop	{r0}
+; CHECK: mov	r12, r3
+; CHECK: mov	r0, r12
 ; CHECK: adcs	r0, r4
 ; CHECK: ldr	r6, [sp, #4]            @ 4-byte Reload
 ; CHECK: str	r0, [r6]

llvm/test/CodeGen/Thumb/urem-seteq-illegal-types.ll

@@ -122,8 +122,8 @@ define <3 x i1> @test_urem_vec(<3 x i11> %X) nounwind {
 ; CHECK-NEXT:    movs r3, #1
 ; CHECK-NEXT:    movs r4, #0
 ; CHECK-NEXT:    cmp r0, #170
-; CHECK-NEXT:    push {r3}
-; CHECK-NEXT:    pop {r0}
+; CHECK-NEXT:    mov r12, r3
+; CHECK-NEXT:    mov r0, r12
 ; CHECK-NEXT:    bhi .LBB4_2
 ; CHECK-NEXT:  @ %bb.1:
 ; CHECK-NEXT:    movs r0, r4
@@ -134,8 +134,8 @@ define <3 x i1> @test_urem_vec(<3 x i11> %X) nounwind {
 ; CHECK-NEXT:    movs r1, #73
 ; CHECK-NEXT:    lsls r1, r1, #23
 ; CHECK-NEXT:    cmp r5, r1
-; CHECK-NEXT:    push {r3}
-; CHECK-NEXT:    pop {r1}
+; CHECK-NEXT:    mov r12, r3
+; CHECK-NEXT:    mov r1, r12
 ; CHECK-NEXT:    bhi .LBB4_4
 ; CHECK-NEXT:  @ %bb.3:
 ; CHECK-NEXT:    movs r1, r4