[inlineasm] Attach readonly and readnone to inline-asm instructions.

Previously, clang/llvm treated inline-asm instructions conservatively,
choosing not to eliminate the instructions or hoisting them out of a loop
even when it was safe to do so. This commit makes changes to attach a
readonly or readnone attribute to an inline-asm instruction, which enables
passes such as LICM and EarlyCSE to move or optimize away the instruction.

rdar://problem/11358192

Differential Revision: http://reviews.llvm.org/D10546

llvm-svn: 241930

1 files changed, 25 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index a12f29534bf..7a0b8a35be0 100644
--- a/clang/lib/CodeGen/CGStmt.cpp
+++ b/clang/lib/CodeGen/CGStmt.cpp
@@ -1852,6 +1852,14 @@ void CodeGenFunction::EmitAsmStmt(const AsmStmt &S) {
   std::vector<llvm::Value*> InOutArgs;
   std::vector<llvm::Type*> InOutArgTypes;
 
+  // An inline asm can be marked readonly if it meets the following conditions:
+  //  - it doesn't have any sideeffects
+  //  - it doesn't clobber memory
+  //  - it doesn't return a value by-reference
+  // It can be marked readnone if it doesn't have any input memory constraints
+  // in addition to meeting the conditions listed above.
+  bool ReadOnly = true, ReadNone = true;
+
   for (unsigned i = 0, e = S.getNumOutputs(); i != e; i++) {
     TargetInfo::ConstraintInfo &Info = OutputConstraintInfos[i];
 
@@ -1915,6 +1923,7 @@ void CodeGenFunction::EmitAsmStmt(const AsmStmt &S) {
       Args.push_back(Dest.getAddress());
       Constraints += "=*";
       Constraints += OutputConstraint;
+      ReadOnly = ReadNone = false;
     }
 
     if (Info.isReadWrite()) {
@@ -1959,6 +1968,9 @@ void CodeGenFunction::EmitAsmStmt(const AsmStmt &S) {
 
     TargetInfo::ConstraintInfo &Info = InputConstraintInfos[i];
 
+    if (Info.allowsMemory())
+      ReadNone = false;
+
     if (!Constraints.empty())
       Constraints += ',';
 
@@ -2023,7 +2035,9 @@ void CodeGenFunction::EmitAsmStmt(const AsmStmt &S) {
   for (unsigned i = 0, e = S.getNumClobbers(); i != e; i++) {
     StringRef Clobber = S.getClobber(i);
 
-    if (Clobber != "memory" && Clobber != "cc")
+    if (Clobber == "memory")
+      ReadOnly = ReadNone = false;
+    else if (Clobber != "cc")
       Clobber = getTarget().getNormalizedGCCRegisterName(Clobber);
 
     if (!Constraints.empty())
@@ -2063,6 +2077,16 @@ void CodeGenFunction::EmitAsmStmt(const AsmStmt &S) {
   Result->addAttribute(llvm::AttributeSet::FunctionIndex,
                        llvm::Attribute::NoUnwind);
 
+  // Attach readnone and readonly attributes.
+  if (!HasSideEffect) {
+    if (ReadNone)
+      Result->addAttribute(llvm::AttributeSet::FunctionIndex,
+                           llvm::Attribute::ReadNone);
+    else if (ReadOnly)
+      Result->addAttribute(llvm::AttributeSet::FunctionIndex,
+                           llvm::Attribute::ReadOnly);
+  }
+
   // Slap the source location of the inline asm into a !srcloc metadata on the
   // call.
   if (const GCCAsmStmt *gccAsmStmt = dyn_cast<GCCAsmStmt>(&S)) {