[RegAllocGreedy] Attempt to split unspillable live intervals

Summary:
Previously, when allocating unspillable live ranges, we would never
attempt to split. We would always bail out and try last ditch graph
recoloring.

This patch changes this by attempting to split all live intervals before
performing recoloring.

This fixes LLVM bug PR14879.

I can't add test cases for any backends other than AVR because none of
them have small enough register classes to trigger the bug.

Reviewers: qcolombet

Subscribers: MatzeB

Differential Revision: https://reviews.llvm.org/D25070

llvm-svn: 282852

2 files changed, 86 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
index 0c93d266004..c46d6071702 100644
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -2556,18 +2556,20 @@ unsigned RAGreedy::selectOrSplitImpl(LiveInterval &VirtReg,
     return 0;
   }
 
+  if (Stage == RS_Split || Stage == RS_Split2) {
+    // Try splitting VirtReg or interferences.
+    unsigned NewVRegSizeBefore = NewVRegs.size();
+    unsigned PhysReg = trySplit(VirtReg, Order, NewVRegs);
+    if (PhysReg || (NewVRegs.size() - NewVRegSizeBefore))
+      return PhysReg;
+  }
+
   // If we couldn't allocate a register from spilling, there is probably some
   // invalid inline assembly. The base class wil report it.
   if (Stage >= RS_Done || !VirtReg.isSpillable())
     return tryLastChanceRecoloring(VirtReg, Order, NewVRegs, FixedRegisters,
                                    Depth);
 
-  // Try splitting VirtReg or interferences.
-  unsigned NewVRegSizeBefore = NewVRegs.size();
-  unsigned PhysReg = trySplit(VirtReg, Order, NewVRegs);
-  if (PhysReg || (NewVRegs.size() - NewVRegSizeBefore))
-    return PhysReg;
-
   // Finally spill VirtReg itself.
   if (EnableDeferredSpilling && getStage(VirtReg) < RS_Memory) {
     // TODO: This is experimental and in particular, we do not model
diff --git a/llvm/test/CodeGen/AVR/high-pressure-on-ptrregs.ll b/llvm/test/CodeGen/AVR/high-pressure-on-ptrregs.ll
new file mode 100644
index 00000000000..41d80863e50
--- /dev/null
+++ b/llvm/test/CodeGen/AVR/high-pressure-on-ptrregs.ll
@@ -0,0 +1,78 @@
+; RUN: llc < %s -march=avr | FileCheck %s
+
+; This tests how LLVM handles IR which puts very high
+; presure on the PTRREGS class for the register allocator.
+;
+; This causes a problem because we only have one small register
+; class for loading and storing from pointers - 'PTRREGS'.
+; One of these registers is also used for the frame pointer, meaning
+; that we only ever have two registers available for these operations.
+;
+; There is an existing bug filed for this issue - PR14879.
+;
+; The specific failure:
+; LLVM ERROR: ran out of registers during register allocation
+;
+; It has been assembled from the following c code:
+;
+; struct ss
+; {
+;   int a;
+;   int b;
+;   int c;
+; };
+;
+; void loop(struct ss *x, struct ss **y, int z)
+; {
+;   int i;
+;   for (i=0; i<z; ++i)
+;   {
+;     x->c += y[i]->b;
+;   }
+; }
+
+%struct.ss = type { i16, i16, i16 }
+
+; CHECK-LABEL: loop
+define void @loop(%struct.ss* %x, %struct.ss** %y, i16 %z) {
+entry:
+  %x.addr = alloca %struct.ss*, align 2
+  %y.addr = alloca %struct.ss**, align 2
+  %z.addr = alloca i16, align 2
+  %i = alloca i16, align 2
+  store %struct.ss* %x, %struct.ss** %x.addr, align 2
+  store %struct.ss** %y, %struct.ss*** %y.addr, align 2
+  store i16 %z, i16* %z.addr, align 2
+  store i16 0, i16* %i, align 2
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %0 = load i16, i16* %i, align 2
+  %1 = load i16, i16* %z.addr, align 2
+  %cmp = icmp slt i16 %0, %1
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %2 = load i16, i16* %i, align 2
+  %3 = load %struct.ss**, %struct.ss*** %y.addr, align 2
+  %arrayidx = getelementptr inbounds %struct.ss*, %struct.ss** %3, i16 %2
+  %4 = load %struct.ss*, %struct.ss** %arrayidx, align 2
+  %b = getelementptr inbounds %struct.ss, %struct.ss* %4, i32 0, i32 1
+  %5 = load i16, i16* %b, align 2
+  %6 = load %struct.ss*, %struct.ss** %x.addr, align 2
+  %c = getelementptr inbounds %struct.ss, %struct.ss* %6, i32 0, i32 2
+  %7 = load i16, i16* %c, align 2
+  %add = add nsw i16 %7, %5
+  store i16 %add, i16* %c, align 2
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %8 = load i16, i16* %i, align 2
+  %inc = add nsw i16 %8, 1
+  store i16 %inc, i16* %i, align 2
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+