[llvm-branch-commits] [llvm] f26bc0d - [RegisterClassInfo] Return non-zero for RC without allocatable reg

[Jinsong Ji via llvm-branch-commits]

Author: Jinsong Ji
Date: 2021-01-05T16:18:34Z
New Revision: f26bc0ddd508edad7e3838850dfcb6b960d6e681

URL: https://github.com/llvm/llvm-project/commit/f26bc0ddd508edad7e3838850dfcb6b960d6e681
DIFF: https://github.com/llvm/llvm-project/commit/f26bc0ddd508edad7e3838850dfcb6b960d6e681.diff

LOG: [RegisterClassInfo] Return non-zero for RC without allocatable reg

In some case, the RC may have 0 allocatable reg.
eg: VRSAVERC in PowerPC, which has only 1 reg, but it is also reserved.

The curreent implementation will keep calling the computePSetLimit because
getRegPressureSetLimit assume computePSetLimit will return a non-zero value.

The fix simply early return the value from TableGen for such special case.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D92907

Added: 
    

Modified: 
    llvm/lib/CodeGen/RegisterClassInfo.cpp
    llvm/test/CodeGen/PowerPC/compute-regpressure.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/RegisterClassInfo.cpp b/llvm/lib/CodeGen/RegisterClassInfo.cpp
index 1523bd4d1649..0488db3d09cb 100644
--- a/llvm/lib/CodeGen/RegisterClassInfo.cpp
+++ b/llvm/lib/CodeGen/RegisterClassInfo.cpp
@@ -188,7 +188,14 @@ unsigned RegisterClassInfo::computePSetLimit(unsigned Idx) const {
   }
   assert(RC && "Failed to find register class");
   compute(RC);
-  unsigned NReserved = RC->getNumRegs() - getNumAllocatableRegs(RC);
-  return TRI->getRegPressureSetLimit(*MF, Idx) -
-         TRI->getRegClassWeight(RC).RegWeight * NReserved;
+  unsigned NAllocatableRegs = getNumAllocatableRegs(RC);
+  unsigned RegPressureSetLimit = TRI->getRegPressureSetLimit(*MF, Idx);
+  // If all the regs are reserved, return raw RegPressureSetLimit.
+  // One example is VRSAVERC in PowerPC.
+  // Avoid returning zero, getRegPressureSetLimit(Idx) assumes computePSetLimit
+  // return non-zero value.
+  if (NAllocatableRegs == 0)
+    return RegPressureSetLimit;
+  unsigned NReserved = RC->getNumRegs() - NAllocatableRegs;
+  return RegPressureSetLimit - TRI->getRegClassWeight(RC).RegWeight * NReserved;
 }

diff  --git a/llvm/test/CodeGen/PowerPC/compute-regpressure.ll b/llvm/test/CodeGen/PowerPC/compute-regpressure.ll
index 7a15e4677267..4ef7fa289381 100644
--- a/llvm/test/CodeGen/PowerPC/compute-regpressure.ll
+++ b/llvm/test/CodeGen/PowerPC/compute-regpressure.ll
@@ -1,7 +1,7 @@
 ; REQUIRES: asserts
 ; RUN: llc -debug-only=regalloc < %s 2>&1 |FileCheck %s --check-prefix=DEBUG
 
-; DEBUG-COUNT-3:         AllocationOrder(VRSAVERC) = [ ]
+; DEBUG-COUNT-1:         AllocationOrder(VRSAVERC) = [ ]
 
 target triple = "powerpc64le-unknown-linux-gnu"