[PPC] Expose the ZERO register as a constant physical register

[Carrot]

Summary

The ZERO register should be exposed as a constant physical register through the interface TargetRegisterInfo::isConstantPhysReg.

[nemanjai]

While this is certainly correct, I am curious what prompted the change. Did it cause a test case failure somewhere? Did it prevent some optimization? If either of those is the case, I imagine we can produce a test case for this patch.

[Carrot]

I'm working on https://reviews.llvm.org/D130919#change-i79krRTVIqHY.

Function MachineRegisterInfo::isConstantPhysReg returns true for a physical register if it satisfies

1. It is a target specific constant register and
2. It is not modified in the current function.

But it only checks defined register operands. A physical register may also be clobbered by a RegMask(function call). So my patch adds checking agains regmasks.

This patch causes f128-aggregates.ll failed. The problem is in MachineCSE pass, when it checks if the following instruction can be CSEed,

%22:vsrc = LXVD2X $zero8, %13:g8rc :: (load (s128) from constant-pool)

A physical register $zero8 is used, then it checks if $zero8 is constant through MachineRegisterInfo::isConstantPhysReg. Because it is not marked as target specific constant register, it continues to check if it is modified in the function. $zero8 is not used as defined register operand, so the original MachineRegisterInfo::isConstantPhysReg returns true for it. But now we also consider RegMask clobbered register. There is a function call in the test, $zero8 is not marked as callee saved register(this is another problem), so now $zero8 is modified by a RegMask, and MachineRegisterInfo::isConstantPhysReg returns false for $zero8.

This patch can fix the failure.

[Carrot]

ping

[arichardson]

In D131445#3727761, @Carrot wrote:

ping

I'd prefer if this wasn't committed and we used my change to auto-generate this method from tablegen (D131962) instead.

[Carrot]

In D131445#3729206, @arichardson wrote:

In D131445#3727761, @Carrot wrote:

ping

I'd prefer if this wasn't committed and we used my change to auto-generate this method from tablegen (D131962) instead.

@arichardson, I like your patch, thanks!

[arichardson]

This change is included in D131962, so this revision can be closed.

llvm/lib/Target/PowerPC/PPCRegisterInfo.h

static const char *stripRegisterPrefix(const char *RegName) {
     }
 
     return RegName;
   }
 
   bool isNonallocatableRegisterCalleeSave(MCRegister Reg) const override {
     return Reg == PPC::LR || Reg == PPC::LR8;
   }
+
+  bool isConstantPhysReg(MCRegister PhysReg) const override {
+    return PhysReg == PPC::ZERO || PhysReg == PPC::ZERO8;
+  }
 };
 
 } // end namespace llvm
 
 #endif