[Clang] Add llvm.loop.unroll.disable to loops with -fno-unroll-loops.

Currently Clang does not respect -fno-unroll-loops during LTO. During
D76916 it was suggested to respect -fno-unroll-loops on a TU basis.

This patch uses the existing llvm.loop.unroll.disable metadata to
disable loop unrolling explicitly for each loop in the TU if
unrolling is disabled. This should ensure that loops from TUs compiled
with -fno-unroll-loops are skipped by the unroller during LTO.

This also means that if a loop from a TU with -fno-unroll-loops
gets inlined into a TU without this option, the loop won't be
unrolled.

Due to the fact that some transforms might drop loop metadata, there
potentially are cases in which we still unroll loops from TUs with
-fno-unroll-loops. I think we should fix those issues rather than
introducing a function attribute to disable loop unrolling during LTO.
Improving the metadata handling will benefit other use cases, like
various loop pragmas, too. And it is an improvement to clang completely
ignoring -fno-unroll-loops during LTO.

If that direction looks good, we can use a similar approach to also
respect -fno-vectorize during LTO, at least for LoopVectorize.

In the future, this might also allow us to remove the UnrollLoops option
LLVM's PassManagerBuilder.

Reviewers: Meinersbur, hfinkel, dexonsmith, tejohnson

Reviewed By: Meinersbur, tejohnson

Differential Revision: https://reviews.llvm.org/D77058

clang/lib/CodeGen/CGLoopInfo.cpp

@@ -10,6 +10,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/Expr.h"
+#include "clang/Basic/CodeGenOptions.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
@@ -573,6 +574,7 @@ void LoopInfoStack::push(BasicBlock *Header, const llvm::DebugLoc &StartLoc,
 }
 
 void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
+                         const clang::CodeGenOptions &CGOpts,
                          ArrayRef<const clang::Attr *> Attrs,
                          const llvm::DebugLoc &StartLoc,
                          const llvm::DebugLoc &EndLoc) {
@@ -753,6 +755,14 @@ void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
     }
   }
 
+  if (CGOpts.OptimizationLevel > 0)
+    // Disable unrolling for the loop, if unrolling is disabled (via
+    // -fno-unroll-loops) and no pragmas override the decision.
+    if (!CGOpts.UnrollLoops &&
+        (StagedAttrs.UnrollEnable == LoopAttributes::Unspecified &&
+         StagedAttrs.UnrollCount == 0))
+      setUnrollState(LoopAttributes::Disable);
+
   /// Stage the attributes.
   push(Header, StartLoc, EndLoc);
 }

clang/lib/CodeGen/CGLoopInfo.h

@@ -29,6 +29,7 @@ class MDNode;
 namespace clang {
 class Attr;
 class ASTContext;
+class CodeGenOptions;
 namespace CodeGen {
 
 /// Attributes that may be specified on loops.
@@ -202,6 +203,7 @@ public:
   /// Begin a new structured loop. Stage attributes from the Attrs list.
   /// The staged attributes are applied to the loop and then cleared.
   void push(llvm::BasicBlock *Header, clang::ASTContext &Ctx,
+            const clang::CodeGenOptions &CGOpts,
             llvm::ArrayRef<const Attr *> Attrs, const llvm::DebugLoc &StartLoc,
             const llvm::DebugLoc &EndLoc);
 

clang/lib/CodeGen/CGStmt.cpp

@@ -728,8 +728,8 @@ void CodeGenFunction::EmitWhileStmt(const WhileStmt &S,
   EmitBlock(LoopHeader.getBlock());
 
   const SourceRange &R = S.getSourceRange();
-  LoopStack.push(LoopHeader.getBlock(), CGM.getContext(), WhileAttrs,
-                 SourceLocToDebugLoc(R.getBegin()),
+  LoopStack.push(LoopHeader.getBlock(), CGM.getContext(), CGM.getCodeGenOpts(),
+                 WhileAttrs, SourceLocToDebugLoc(R.getBegin()),
                  SourceLocToDebugLoc(R.getEnd()));
 
   // Create an exit block for when the condition fails, which will
@@ -830,7 +830,7 @@ void CodeGenFunction::EmitDoStmt(const DoStmt &S,
   EmitBlock(LoopCond.getBlock());
 
   const SourceRange &R = S.getSourceRange();
-  LoopStack.push(LoopBody, CGM.getContext(), DoAttrs,
+  LoopStack.push(LoopBody, CGM.getContext(), CGM.getCodeGenOpts(), DoAttrs,
                  SourceLocToDebugLoc(R.getBegin()),
                  SourceLocToDebugLoc(R.getEnd()));
 
@@ -888,7 +888,7 @@ void CodeGenFunction::EmitForStmt(const ForStmt &S,
   EmitBlock(CondBlock);
 
   const SourceRange &R = S.getSourceRange();
-  LoopStack.push(CondBlock, CGM.getContext(), ForAttrs,
+  LoopStack.push(CondBlock, CGM.getContext(), CGM.getCodeGenOpts(), ForAttrs,
                  SourceLocToDebugLoc(R.getBegin()),
                  SourceLocToDebugLoc(R.getEnd()));
 
@@ -989,7 +989,7 @@ CodeGenFunction::EmitCXXForRangeStmt(const CXXForRangeStmt &S,
   EmitBlock(CondBlock);
 
   const SourceRange &R = S.getSourceRange();
-  LoopStack.push(CondBlock, CGM.getContext(), ForAttrs,
+  LoopStack.push(CondBlock, CGM.getContext(), CGM.getCodeGenOpts(), ForAttrs,
                  SourceLocToDebugLoc(R.getBegin()),
                  SourceLocToDebugLoc(R.getEnd()));
 

clang/test/CodeGenCXX/fno-unroll-loops-metadata.cpp

@@ -0,0 +1,48 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -std=c++11 -emit-llvm -o - %s -O0 -disable-llvm-optzns -fno-unroll-loops | FileCheck --check-prefix=NO_UNROLL_MD %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -std=c++11 -emit-llvm -o - %s -O1 -disable-llvm-optzns -fno-unroll-loops | FileCheck --check-prefix=UNROLL_DISABLED_MD %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -std=c++11 -emit-llvm -o - %s -O2 -disable-llvm-optzns -fno-unroll-loops | FileCheck --check-prefix=UNROLL_DISABLED_MD %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -std=c++11 -emit-llvm -o - %s -O3 -disable-llvm-optzns -fno-unroll-loops | FileCheck --check-prefix=UNROLL_DISABLED_MD %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -std=c++11 -emit-llvm -o - %s -O3 -disable-llvm-optzns | FileCheck --check-prefix=NO_UNROLL_MD %s
+
+// NO_UNROLL_MD-NOT: llvm.loop
+
+// Verify unroll.disable metadata is added to while loop with -fno-unroll-loops
+// and optlevel > 0.
+void while_test(int *List, int Length) {
+  // UNROLL_DISABLED_MD: define {{.*}} @_Z10while_test
+  int i = 0;
+
+  while (i < Length) {
+    // UNROLL_DISABLED_MD: br label {{.*}}, !llvm.loop ![[LOOP_1:.*]]
+    List[i] = i * 2;
+    i++;
+  }
+}
+
+// Verify unroll.disable metadata is added to do-while loop with
+// -fno-unroll-loops and optlevel > 0.
+void do_test(int *List, int Length) {
+  // UNROLL_DISABLED_MD: define {{.*}} @_Z7do_test
+  int i = 0;
+
+  do {
+    // UNROLL_DISABLED_MD: br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[LOOP_2:.*]]
+    List[i] = i * 2;
+    i++;
+  } while (i < Length);
+}
+
+// Verify unroll.disable metadata is added to while loop with -fno-unroll-loops
+// and optlevel > 0.
+void for_test(int *List, int Length) {
+  // UNROLL_DISABLED_MD: define {{.*}} @_Z8for_test
+  for (int i = 0; i < Length; i++) {
+    // UNROLL_DISABLED_MD: br label {{.*}}, !llvm.loop ![[LOOP_3:.*]]
+    List[i] = i * 2;
+  }
+}
+
+// UNROLL_DISABLED_MD: ![[LOOP_1]] = distinct !{![[LOOP_1]], ![[UNROLL_DISABLE:.*]]}
+// UNROLL_DISABLED_MD: ![[UNROLL_DISABLE]] = !{!"llvm.loop.unroll.disable"}
+// UNROLL_DISABLED_MD: ![[LOOP_2]] = distinct !{![[LOOP_2:.*]], ![[UNROLL_DISABLE:.*]]}
+// UNROLL_DISABLED_MD: ![[LOOP_3]] = distinct !{![[LOOP_3]], ![[UNROLL_DISABLE:.*]]}

clang/test/CodeGenCXX/pragma-unroll.cpp

@@ -1,5 +1,8 @@
 // RUN: %clang_cc1 -triple x86_64-apple-darwin -std=c++11 -emit-llvm -o - %s | FileCheck %s
 
+// Check that passing -fno-unroll-loops does not impact the decision made using pragmas.
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -std=c++11 -emit-llvm -o - -O1 -disable-llvm-optzns -fno-unroll-loops %s | FileCheck %s
+
 // Verify while loop is recognized after unroll pragma.
 void while_test(int *List, int Length) {
   // CHECK: define {{.*}} @_Z10while_test