From fc7676769251a27cbbc6d40d68f04bfe38511a5b Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@qt.io>
Date: Thu, 22 Sep 2022 11:39:44 +0200
Subject: Long live Q_UNREACHABLE_RETURN()!

This is a combination of Q_UNREACHABLE() with a return statement.

ATM, the return statement is unconditionally included. If we notice
that some compilers warn about return after __builtin_unreachable(),
then we can map Q_UNREACHABLE_RETURN(...) to Q_UNREACHABLE() without
having to touch all the code that uses explicit Q_UNREACHABLE() +
return.

The fact that Boost has BOOST_UNREACHABLE_RETURN() indicates that
there are compilers that complain about a lack of return after
Q_UNREACHABLE (we know that MSVC, ICC, and GHS are among them), as
well as compilers that complained about a return being present
(Coverity). Take this opportunity to properly adapt to Coverity, by
leaving out the return statement on this compiler.

Apply the macro around the code base, using a clang-tidy transformer
rule:

    const std::string unr = "unr", val = "val", ret = "ret";
    auto makeUnreachableReturn = cat("Q_UNREACHABLE_RETURN(",
                                    ifBound(val, cat(node(val)), cat("")),
                                    ")");
    auto ignoringSwitchCases = [](auto stmt) {
        return anyOf(stmt, switchCase(subStmt(stmt)));
    };

    makeRule(
       stmt(ignoringSwitchCases(stmt(isExpandedFromMacro("Q_UNREACHABLE")).bind(unr)),
            nextStmt(returnStmt(optionally(hasReturnValue(expr().bind(val)))).bind(ret))),
       {changeTo(node(unr), cat(makeUnreachableReturn,
                                ";")),  // TODO: why is the ; lost w/o this?
        changeTo(node(ret), cat(""))},
       cat("use ", makeUnreachableReturn))
    );

where nextStmt() is copied from some upstream clang-tidy check's
private implementation and subStmt() is a private matcher that gives
access to SwitchCase's SubStmt.

A.k.a. qt-use-unreachable-return.

There were some false positives, suppressed them with NOLINTNEXTLINE.

They're not really false positiives, it's just that Clang sees the
world in one way and if conditonal compilation (#if) differs for other
compilers, Clang doesn't know better. This is an artifact of matching
two consecutive statements.

I haven't figured out how to remove the empty line left by the
deletion of the return statement, if it, indeed, was on a separate
line, so post-processed the patch to remove all the lines matching
^\+ *$ from the diff:

  git commit -am meep
  git reset --hard HEAD^
  git diff HEAD..HEAD@{1} | sed '/^\+ *$/d' | recountdiff - | patch -p1

[ChangeLog][QtCore][QtAssert] Added Q_UNREACHABLE_RETURN() macro.

Change-Id: I9782939f16091c964f25b7826e1c0dbd13a71305
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/gui/rhi/qrhigles2.cpp | 51 ++++++++++++++++-------------------------------
 1 file changed, 17 insertions(+), 34 deletions(-)

(limited to 'src/gui/rhi/qrhigles2.cpp')

diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index a45915495e..a8b7919647 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -1232,8 +1232,7 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
     case QRhi::NonFillPolygonMode:
         return !caps.gles;
     default:
-        Q_UNREACHABLE();
-        return false;
+        Q_UNREACHABLE_RETURN(false);
     }
 }
 
@@ -1271,8 +1270,7 @@ int QRhiGles2::resourceLimit(QRhi::ResourceLimit limit) const
     case QRhi::MaxVertexOutputs:
         return caps.maxVertexOutputs;
     default:
-        Q_UNREACHABLE();
-        return 0;
+        Q_UNREACHABLE_RETURN(0);
     }
 }
 
@@ -2356,8 +2354,7 @@ static inline GLenum toGlTopology(QRhiGraphicsPipeline::Topology t)
     case QRhiGraphicsPipeline::Patches:
         return GL_PATCHES;
     default:
-        Q_UNREACHABLE();
-        return GL_TRIANGLES;
+        Q_UNREACHABLE_RETURN(GL_TRIANGLES);
     }
 }
 
@@ -2369,8 +2366,7 @@ static inline GLenum toGlCullMode(QRhiGraphicsPipeline::CullMode c)
     case QRhiGraphicsPipeline::Back:
         return GL_BACK;
     default:
-        Q_UNREACHABLE();
-        return GL_BACK;
+        Q_UNREACHABLE_RETURN(GL_BACK);
     }
 }
 
@@ -2382,8 +2378,7 @@ static inline GLenum toGlFrontFace(QRhiGraphicsPipeline::FrontFace f)
     case QRhiGraphicsPipeline::CW:
         return GL_CW;
     default:
-        Q_UNREACHABLE();
-        return GL_CCW;
+        Q_UNREACHABLE_RETURN(GL_CCW);
     }
 }
 
@@ -2427,8 +2422,7 @@ static inline GLenum toGlBlendFactor(QRhiGraphicsPipeline::BlendFactor f)
         qWarning("Unsupported blend factor %d", f);
         return GL_ZERO;
     default:
-        Q_UNREACHABLE();
-        return GL_ZERO;
+        Q_UNREACHABLE_RETURN(GL_ZERO);
     }
 }
 
@@ -2446,8 +2440,7 @@ static inline GLenum toGlBlendOp(QRhiGraphicsPipeline::BlendOp op)
     case QRhiGraphicsPipeline::Max:
         return GL_MAX;
     default:
-        Q_UNREACHABLE();
-        return GL_FUNC_ADD;
+        Q_UNREACHABLE_RETURN(GL_FUNC_ADD);
     }
 }
 
@@ -2471,8 +2464,7 @@ static inline GLenum toGlCompareOp(QRhiGraphicsPipeline::CompareOp op)
     case QRhiGraphicsPipeline::Always:
         return GL_ALWAYS;
     default:
-        Q_UNREACHABLE();
-        return GL_ALWAYS;
+        Q_UNREACHABLE_RETURN(GL_ALWAYS);
     }
 }
 
@@ -2496,8 +2488,7 @@ static inline GLenum toGlStencilOp(QRhiGraphicsPipeline::StencilOp op)
     case QRhiGraphicsPipeline::DecrementAndWrap:
         return GL_DECR_WRAP;
     default:
-        Q_UNREACHABLE();
-        return GL_KEEP;
+        Q_UNREACHABLE_RETURN(GL_KEEP);
     }
 }
 
@@ -2509,8 +2500,7 @@ static inline GLenum toGlPolygonMode(QRhiGraphicsPipeline::PolygonMode mode)
     case QRhiGraphicsPipeline::PolygonMode::Line:
         return GL_LINE;
     default:
-        Q_UNREACHABLE();
-        return GL_FILL;
+        Q_UNREACHABLE_RETURN(GL_FILL);
     }
 }
 
@@ -2528,8 +2518,7 @@ static inline GLenum toGlMinFilter(QRhiSampler::Filter f, QRhiSampler::Filter m)
         else
             return m == QRhiSampler::Nearest ? GL_LINEAR_MIPMAP_NEAREST : GL_LINEAR_MIPMAP_LINEAR;
     default:
-        Q_UNREACHABLE();
-        return GL_LINEAR;
+        Q_UNREACHABLE_RETURN(GL_LINEAR);
     }
 }
 
@@ -2541,8 +2530,7 @@ static inline GLenum toGlMagFilter(QRhiSampler::Filter f)
     case QRhiSampler::Linear:
         return GL_LINEAR;
     default:
-        Q_UNREACHABLE();
-        return GL_LINEAR;
+        Q_UNREACHABLE_RETURN(GL_LINEAR);
     }
 }
 
@@ -2556,8 +2544,7 @@ static inline GLenum toGlWrapMode(QRhiSampler::AddressMode m)
     case QRhiSampler::Mirror:
         return GL_MIRRORED_REPEAT;
     default:
-        Q_UNREACHABLE();
-        return GL_CLAMP_TO_EDGE;
+        Q_UNREACHABLE_RETURN(GL_CLAMP_TO_EDGE);
     }
 }
 
@@ -2581,8 +2568,7 @@ static inline GLenum toGlTextureCompareFunc(QRhiSampler::CompareOp op)
     case QRhiSampler::Always:
         return GL_ALWAYS;
     default:
-        Q_UNREACHABLE();
-        return GL_NEVER;
+        Q_UNREACHABLE_RETURN(GL_NEVER);
     }
 }
 
@@ -4275,8 +4261,7 @@ static inline GLenum toGlShaderType(QRhiShaderStage::Type type)
     case QRhiShaderStage::Compute:
         return GL_COMPUTE_SHADER;
     default:
-        Q_UNREACHABLE();
-        return GL_VERTEX_SHADER;
+        Q_UNREACHABLE_RETURN(GL_VERTEX_SHADER);
     }
 }
 
@@ -4558,8 +4543,7 @@ static inline QShader::Stage toShaderStage(QRhiShaderStage::Type type)
     case QRhiShaderStage::Compute:
         return QShader::ComputeStage;
     default:
-        Q_UNREACHABLE();
-        return QShader::VertexStage;
+        Q_UNREACHABLE_RETURN(QShader::VertexStage);
     }
 }
 
@@ -5526,8 +5510,7 @@ bool QGles2GraphicsPipeline::create()
         default:
             break;
         }
-        Q_UNREACHABLE();
-        return VtxIdx;
+        Q_UNREACHABLE_RETURN(VtxIdx);
     };
     QShaderDescription desc[LastIdx];
     QShader::SeparateToCombinedImageSamplerMappingList samplerMappingList[LastIdx];
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