[AMDGPU] Fix gcc 9.3.0 warning from -Wtype-limits

[vabridgers]

Summary

Fixes this warning when using gcc 9.3.0

lib/Target/AMDGPU/R600GenSubtargetInfo.inc:282:62: warning: comparison
of unsigned expression < 0 is always false [-Wtype-limits]
if (Bits[R600::FeatureLocalMemorySize0] && LocalMemorySize <0)

LocalMemorySize = 0;

[dblaikie]

Can we fix the inc file instead?

[vabridgers]

Agreed it's better to fix in the inc file. This is generated, any ideas where the source is for the generated file? Perhaps a maintainer could point me in the right direction?

[foad]

In D108863#2975134, @vabridgers wrote:

Agreed it's better to fix in the inc file. This is generated, any ideas where the source is for the generated file? Perhaps a maintainer could point me in the right direction?

The code that provokes the warning was generated by SubtargetEmitter::ParseFeaturesFunction in utils/TableGen/SubtargetEmitter.cpp. I'm not sure whether this code knows the exact type of Attribute, e.g. whether it's int or unsigned.

Maybe instead of:

if (Bits[R600::FeatureLocalMemorySize0] && LocalMemorySize < 0) LocalMemorySize = 0;

it could generate:

if (Bits[R600::FeatureLocalMemorySize0]) LocalMemorySize = std::max(LocalMemorySize, 0);

and the warning would go away?

llvm/lib/Target/AMDGPU/R600Subtarget.cpp

 //===-- R600Subtarget.cpp - R600 Subtarget Information --------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 /// \file
 /// Implements the R600 specific subclass of TargetSubtarget.
 //
 //===----------------------------------------------------------------------===//
 
 #include "R600Subtarget.h"
 #include "MCTargetDesc/R600MCTargetDesc.h"
 
 using namespace llvm;
 
 #define DEBUG_TYPE "r600-subtarget"
 
 #define GET_SUBTARGETINFO_TARGET_DESC
 #define GET_SUBTARGETINFO_CTOR
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wtype-limits"
 #include "R600GenSubtargetInfo.inc"
+#pragma GCC diagnostic pop
 
 R600Subtarget::R600Subtarget(const Triple &TT, StringRef GPU, StringRef FS,
                              const TargetMachine &TM)
     : R600GenSubtargetInfo(TT, GPU, /*TuneCPU*/ GPU, FS), AMDGPUSubtarget(TT),
       InstrInfo(*this),
       FrameLowering(TargetFrameLowering::StackGrowsUp, getStackAlignment(), 0),
       FMA(false), CaymanISA(false), CFALUBug(false), HasVertexCache(false),
       R600ALUInst(false), FP64(false), TexVTXClauseSize(0), Gen(R600),