[ARM] Alter the register allocation order for optsize on Thumb2

[dmgreen]

Summary

Currently in Arm code, we allocate LR first, under the assumption that
it needs to be saved anyway. Unfortunately this has the disadvantage
that it will require any instructions using it to be the longer thumb2
instructions, not the shorter thumb1 ones.

This switches the order when we are optimising for minsize, returning to
the default order so that more lower registers can be used. It can end
up requiring more pushed registers, but on average produces smaller code.

[efriedma]

This isn't the first time this has come up; see https://reviews.llvm.org/D30324 . I guess changing the allocation order for lr, but not ip, makes this version simpler?

[dmgreen]

Hello

I had not seen that, thanks for pointing to it. Yes, this is a bit simpler, not trying to deal with r12. Just using the default allocation order only gets us so far, but it seems like a simple enough change. I had tried a few different orderings around using r4/r7 before lr, for example, as they are often spilled in pairs, but this seemed to give the best results for the codebases I tried.

I would say that this patch get us a little bit of codesize in many places, adding up to a good overall gain. Compared to all the other codefolding/libcall style changes we might try to make (that would occur in less places), this seems like an easier win for a simple change.

[efriedma]

I guess this is fine as an incremental change. LGTM

lib/Target/ARM/ARMRegisterInfo.td

274

80 cols.

lib/Target/ARM/ARMRegisterInfo.td

 // r7  == Frame Pointer (thumb-style backtraces)
 // r9  == May be reserved as Thread Register
 // r11 == Frame Pointer (arm-style backtraces)
 // r10 == Stack Limit
 //
 def GPR : RegisterClass<"ARM", [i32], 32, (add (sequence "R%u", 0, 12),
                                                SP, LR, PC)> {
   // Allocate LR as the first CSR since it is always saved anyway.
+  // For Thumb2, using LR would force 32bit Thumb2 instructions, not the smaller
+  // Thumb1 ones. It is a little better for codesize on average to use the
+  // default order.
   // For Thumb1 mode, we don't want to allocate hi regs at all, as we don't
   // know how to spill them. If we make our prologue/epilogue code smarter at
   // some point, we can go back to using the above allocation orders for the
   // Thumb1 instructions that know how to use hi regs.
   let AltOrders = [(add LR, GPR), (trunc GPR, 8)];
   let AltOrderSelect = [{
-      return 1 + MF.getSubtarget<ARMSubtarget>().isThumb1Only();
+      if (MF.getSubtarget<ARMSubtarget>().isThumb1Only())
+        return 2;
+      if (MF.getSubtarget<ARMSubtarget>().isThumb2() && MF.getFunction().optForMinSize())
+        return 0;
+      return 1;
   }];
   let DiagnosticString = "operand must be a register in range [r0, r15]";
 }
 
 // GPRs without the PC.  Some ARM instructions do not allow the PC in
 // certain operand slots, particularly as the destination.  Primarily
 // useful for disassembly.
 def GPRnopc : RegisterClass<"ARM", [i32], 32, (sub GPR, PC)> {
   let AltOrders = [(add LR, GPRnopc), (trunc GPRnopc, 8)];
   let AltOrderSelect = [{
-      return 1 + MF.getSubtarget<ARMSubtarget>().isThumb1Only();
+      if (MF.getSubtarget<ARMSubtarget>().isThumb1Only())
+        return 2;
+      if (MF.getSubtarget<ARMSubtarget>().isThumb2() && MF.getFunction().optForMinSize())
+        return 0;
+      return 1;
   }];
   let DiagnosticString = "operand must be a register in range [r0, r14]";
 }
 
 // GPRs without the PC but with APSR. Some instructions allow accessing the
 // APSR, while actually encoding PC in the register field. This is useful
 // for assembly and disassembly only.
 def GPRwithAPSR : RegisterClass<"ARM", [i32], 32, (add (sub GPR, PC), APSR_NZCV)> {
   let AltOrders = [(add LR, GPRnopc), (trunc GPRnopc, 8)];
   let AltOrderSelect = [{
-      return 1 + MF.getSubtarget<ARMSubtarget>().isThumb1Only();
+      if (MF.getSubtarget<ARMSubtarget>().isThumb1Only())
+        return 2;
+      if (MF.getSubtarget<ARMSubtarget>().isThumb2() && MF.getFunction().optForMinSize())
+        return 0;
+      return 1;
   }];
   let DiagnosticString = "operand must be a register in range [r0, r14] or apsr_nzcv";
 }
 
 // GPRsp - Only the SP is legal. Used by Thumb1 instructions that want the
 // implied SP argument list.
 // FIXME: It would be better to not use this at all and refactor the
 // instructions to not have SP an an explicit argument. That makes
 // frame index resolution a bit trickier, though.
 def GPRsp : RegisterClass<"ARM", [i32], 32, (add SP)> {
   let DiagnosticString = "operand must be a register sp";
 }
 
 // restricted GPR register class. Many Thumb2 instructions allow the full
 // register range for operands, but have undefined behaviours when PC
 // or SP (R13 or R15) are used. The ARM ISA refers to these operands
 // via the BadReg() pseudo-code description.
 def rGPR : RegisterClass<"ARM", [i32], 32, (sub GPR, SP, PC)> {
   let AltOrders = [(add LR, rGPR), (trunc rGPR, 8)];
   let AltOrderSelect = [{
-      return 1 + MF.getSubtarget<ARMSubtarget>().isThumb1Only();
+      if (MF.getSubtarget<ARMSubtarget>().isThumb1Only())
+        return 2;
+      if (MF.getSubtarget<ARMSubtarget>().isThumb2() && MF.getFunction().optForMinSize())
+        return 0;
+      return 1;
   }];
   let DiagnosticType = "rGPR";
 }
 
 // Thumb registers are R0-R7 normally. Some instructions can still use
 // the general GPR register class above (MOV, e.g.)
 def tGPR : RegisterClass<"ARM", [i32], 32, (trunc GPR, 8)> {
   let DiagnosticString = "operand must be a register in range [r0, r7]";

test/CodeGen/Thumb2/reg-order.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s -mtriple=thumbv7m-none-eabi | FileCheck %s
 
 
 define i32 @test(i32 %a, i32 %b, i32 %c, i32 %d) #0 {
 ; CHECK-LABEL: test:
 ; CHECK:       @ %bb.0: @ %entry
-; CHECK-NEXT:    .save {r7, lr}
-; CHECK-NEXT:    push {r7, lr}
-; CHECK-NEXT:    add.w lr, r3, r0
+; CHECK-NEXT:    .save {r4, lr}
+; CHECK-NEXT:    push {r4, lr}
+; CHECK-NEXT:    adds r4, r3, r0
 ; CHECK-NEXT:    add.w r12, r2, r1
 ; CHECK-NEXT:    add r0, r1
 ; CHECK-NEXT:    adds r1, r3, r2
-; CHECK-NEXT:    mul r12, lr, r12
-; CHECK-NEXT:    mla r0, r1, r0, r12
-; CHECK-NEXT:    pop {r7, pc}
+; CHECK-NEXT:    mul r4, r4, r12
+; CHECK-NEXT:    mla r0, r1, r0, r4
+; CHECK-NEXT:    pop {r4, pc}
 entry:
   %add = add nsw i32 %b, %a
   %add1 = add nsw i32 %d, %c
   %mul = mul nsw i32 %add1, %add
   %add2 = add nsw i32 %d, %a
   %add3 = add nsw i32 %c, %b
   %mul4 = mul nsw i32 %add2, %add3
   %add5 = add nsw i32 %mul, %mul4
   ret i32 %add5
 }
 
 define void @loop(i32 %I, i8* %A, i8* %B) #0 {
 ; CHECK-LABEL: loop:
 ; CHECK:       @ %bb.0: @ %entry
-; CHECK-NEXT:    .save {r4, r5, r6, lr}
-; CHECK-NEXT:    push {r4, r5, r6, lr}
+; CHECK-NEXT:    .save {r4, r5, r6, r7, lr}
+; CHECK-NEXT:    push {r4, r5, r6, r7, lr}
 ; CHECK-NEXT:    mov.w r12, #0
 ; CHECK-NEXT:    b .LBB1_2
 ; CHECK-NEXT:  .LBB1_1: @ %for.body
 ; CHECK-NEXT:    @ in Loop: Header=BB1_2 Depth=1
-; CHECK-NEXT:    add.w r3, r12, r12, lsl #1
-; CHECK-NEXT:    add.w lr, r2, r12, lsl #2
-; CHECK-NEXT:    add r3, r1
+; CHECK-NEXT:    add.w r4, r12, r12, lsl #1
+; CHECK-NEXT:    add.w r3, r2, r12, lsl #2
+; CHECK-NEXT:    add r4, r1
 ; CHECK-NEXT:    add.w r12, r12, #1
-; CHECK-NEXT:    ldrsb.w r5, [r3, #2]
-; CHECK-NEXT:    ldrsb.w r4, [r3]
-; CHECK-NEXT:    mov r6, r5
-; CHECK-NEXT:    cmp r4, r5
+; CHECK-NEXT:    ldrsb.w r6, [r4, #2]
+; CHECK-NEXT:    ldrsb.w r5, [r4]
+; CHECK-NEXT:    mov r7, r6
+; CHECK-NEXT:    cmp r5, r6
 ; CHECK-NEXT:    it gt
-; CHECK-NEXT:    movgt r6, r4
-; CHECK-NEXT:    ldrsb.w r3, [r3, #1]
-; CHECK-NEXT:    cmp r6, r3
+; CHECK-NEXT:    movgt r7, r5
+; CHECK-NEXT:    ldrsb.w r4, [r4, #1]
+; CHECK-NEXT:    cmp r7, r4
 ; CHECK-NEXT:    it le
-; CHECK-NEXT:    movle r6, r3
-; CHECK-NEXT:    subs r3, r6, r3
-; CHECK-NEXT:    subs r5, r6, r5
-; CHECK-NEXT:    strb.w r5, [lr, #3]
-; CHECK-NEXT:    strb.w r3, [lr, #2]
-; CHECK-NEXT:    subs r3, r6, r4
-; CHECK-NEXT:    strb.w r3, [lr, #1]
-; CHECK-NEXT:    mvns r3, r6
-; CHECK-NEXT:    strb.w r3, [lr]
+; CHECK-NEXT:    movle r7, r4
+; CHECK-NEXT:    subs r4, r7, r4
+; CHECK-NEXT:    subs r6, r7, r6
+; CHECK-NEXT:    strb r6, [r3, #3]
+; CHECK-NEXT:    strb r4, [r3, #2]
+; CHECK-NEXT:    subs r4, r7, r5
+; CHECK-NEXT:    strb r4, [r3, #1]
+; CHECK-NEXT:    mvns r4, r7
+; CHECK-NEXT:    strb r4, [r3]
 ; CHECK-NEXT:  .LBB1_2: @ %for.cond
 ; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
 ; CHECK-NEXT:    cmp r12, r0
 ; CHECK-NEXT:    blt .LBB1_1
 ; CHECK-NEXT:  @ %bb.3: @ %for.cond.cleanup
-; CHECK-NEXT:    pop {r4, r5, r6, pc}
+; CHECK-NEXT:    pop {r4, r5, r6, r7, pc}
 entry:
   br label %for.cond
 
 for.cond:                                         ; preds = %for.body, %entry
   %A.addr.0 = phi i8* [ %A, %entry ], [ %incdec.ptr2, %for.body ]
   %B.addr.0 = phi i8* [ %B, %entry ], [ %incdec.ptr47, %for.body ]
   %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
   %cmp = icmp slt i32 %i.0, %I