[CodeGen] emit inline asm clobber list warnings for reserved

Summary:
Currently, in line with GCC, when specifying reserved registers like sp or pc on an inline asm() clobber list, we don't always preserve the original value across the statement. And in general, overwriting reserved registers can have surprising results.

For example:


```
extern int bar(int[]);

int foo(int i) {
  int a[i]; // VLA
  asm volatile(
      "mov r7, #1"
    :
    :
    : "r7"
  );

  return 1 + bar(a);
}
```

Compiled for thumb, this gives:
```
$ clang --target=arm-arm-none-eabi -march=armv7a -c test.c -o - -S -O1 -mthumb
...
foo:
        .fnstart
@ %bb.0:                                @ %entry
        .save   {r4, r5, r6, r7, lr}
        push    {r4, r5, r6, r7, lr}
        .setfp  r7, sp, #12
        add     r7, sp, #12
        .pad    #4
        sub     sp, #4
        movs    r1, #7
        add.w   r0, r1, r0, lsl #2
        bic     r0, r0, #7
        sub.w   r0, sp, r0
        mov     sp, r0
        @app
        mov.w   r7, #1
        @NO_APP
        bl      bar
        adds    r0, #1
        sub.w   r4, r7, #12
        mov     sp, r4
        pop     {r4, r5, r6, r7, pc}
...
```

r7 is used as the frame pointer for thumb targets, and this function needs to restore the SP from the FP because of the variable-length stack allocation a. r7 is clobbered by the inline assembly (and r7 is included in the clobber list), but LLVM does not preserve the value of the frame pointer across the assembly block.

This type of behavior is similar to GCC's and has been discussed on the bugtracker: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=11807 . No consensus seemed to have been reached on the way forward.  Clang behavior has briefly been discussed on the CFE mailing (starting here: http://lists.llvm.org/pipermail/cfe-dev/2018-July/058392.html). I've opted for following Eli Friedman's advice to print warnings when there are reserved registers on the clobber list so as not to diverge from GCC behavior for now.

The patch uses MachineRegisterInfo's target-specific knowledge of reserved registers, just before we convert the inline asm string in the AsmPrinter.

If we find a reserved register, we print a warning:
```
repro.c:6:7: warning: inline asm clobber list contains reserved registers: R7 [-Winline-asm]
      "mov r7, #1"
      ^
```

Reviewers: eli.friedman, olista01, javed.absar, efriedma

Reviewed By: efriedma

Subscribers: efriedma, eraman, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D49727

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339257 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/AsmPrinter.h

@@ -631,6 +631,11 @@ class AsmPrinter : public MachineFunctionPass {
   /// inline asm.
   void EmitInlineAsm(const MachineInstr *MI) const;
 
+  /// Add inline assembly info to the diagnostics machinery, so we can
+  /// emit file and position info. Returns SrcMgr memory buffer position.
+  unsigned addInlineAsmDiagBuffer(StringRef AsmStr,
+                                  const MDNode *LocMDNode) const;
+
   //===------------------------------------------------------------------===//
   // Internal Implementation Details
   //===------------------------------------------------------------------===//

lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

@@ -71,6 +71,42 @@ static void srcMgrDiagHandler(const SMDiagnostic &Diag, void *diagInfo) {
   DiagInfo->DiagHandler(Diag, DiagInfo->DiagContext, LocCookie);
 }
 
+unsigned AsmPrinter::addInlineAsmDiagBuffer(StringRef AsmStr,
+                                            const MDNode *LocMDNode) const {
+  if (!DiagInfo) {
+    DiagInfo = make_unique<SrcMgrDiagInfo>();
+
+    MCContext &Context = MMI->getContext();
+    Context.setInlineSourceManager(&DiagInfo->SrcMgr);
+
+    LLVMContext &LLVMCtx = MMI->getModule()->getContext();
+    if (LLVMCtx.getInlineAsmDiagnosticHandler()) {
+      DiagInfo->DiagHandler = LLVMCtx.getInlineAsmDiagnosticHandler();
+      DiagInfo->DiagContext = LLVMCtx.getInlineAsmDiagnosticContext();
+      DiagInfo->SrcMgr.setDiagHandler(srcMgrDiagHandler, DiagInfo.get());
+    }
+  }
+
+  SourceMgr &SrcMgr = DiagInfo->SrcMgr;
+
+  std::unique_ptr<MemoryBuffer> Buffer;
+  // The inline asm source manager will outlive AsmStr, so make a copy of the
+  // string for SourceMgr to own.
+  Buffer = MemoryBuffer::getMemBufferCopy(AsmStr, "<inline asm>");
+
+  // Tell SrcMgr about this buffer, it takes ownership of the buffer.
+  unsigned BufNum = SrcMgr.AddNewSourceBuffer(std::move(Buffer), SMLoc());
+
+  // Store LocMDNode in DiagInfo, using BufNum as an identifier.
+  if (LocMDNode) {
+    DiagInfo->LocInfos.resize(BufNum);
+    DiagInfo->LocInfos[BufNum - 1] = LocMDNode;
+  }
+
+  return BufNum;
+}
+
+
 /// EmitInlineAsm - Emit a blob of inline asm to the output streamer.
 void AsmPrinter::EmitInlineAsm(StringRef Str, const MCSubtargetInfo &STI,
                                const MCTargetOptions &MCOptions,
@@ -98,39 +134,11 @@ void AsmPrinter::EmitInlineAsm(StringRef Str, const MCSubtargetInfo &STI,
     return;
   }
 
-  if (!DiagInfo) {
-    DiagInfo = make_unique<SrcMgrDiagInfo>();
+  unsigned BufNum = addInlineAsmDiagBuffer(Str, LocMDNode);
+  DiagInfo->SrcMgr.setIncludeDirs(MCOptions.IASSearchPaths);
 
-    MCContext &Context = MMI->getContext();
-    Context.setInlineSourceManager(&DiagInfo->SrcMgr);
-
-    LLVMContext &LLVMCtx = MMI->getModule()->getContext();
-    if (LLVMCtx.getInlineAsmDiagnosticHandler()) {
-      DiagInfo->DiagHandler = LLVMCtx.getInlineAsmDiagnosticHandler();
-      DiagInfo->DiagContext = LLVMCtx.getInlineAsmDiagnosticContext();
-      DiagInfo->SrcMgr.setDiagHandler(srcMgrDiagHandler, DiagInfo.get());
-    }
-  }
-
-  SourceMgr &SrcMgr = DiagInfo->SrcMgr;
-  SrcMgr.setIncludeDirs(MCOptions.IASSearchPaths);
-
-  std::unique_ptr<MemoryBuffer> Buffer;
-  // The inline asm source manager will outlive Str, so make a copy of the
-  // string for SourceMgr to own.
-  Buffer = MemoryBuffer::getMemBufferCopy(Str, "<inline asm>");
-
-  // Tell SrcMgr about this buffer, it takes ownership of the buffer.
-  unsigned BufNum = SrcMgr.AddNewSourceBuffer(std::move(Buffer), SMLoc());
-
-  // Store LocMDNode in DiagInfo, using BufNum as an identifier.
-  if (LocMDNode) {
-    DiagInfo->LocInfos.resize(BufNum);
-    DiagInfo->LocInfos[BufNum-1] = LocMDNode;
-  }
-
-  std::unique_ptr<MCAsmParser> Parser(
-      createMCAsmParser(SrcMgr, OutContext, *OutStreamer, *MAI, BufNum));
+  std::unique_ptr<MCAsmParser> Parser(createMCAsmParser(
+          DiagInfo->SrcMgr, OutContext, *OutStreamer, *MAI, BufNum));
 
   // Do not use assembler-level information for parsing inline assembly.
   OutStreamer->setUseAssemblerInfoForParsing(false);
@@ -519,6 +527,44 @@ void AsmPrinter::EmitInlineAsm(const MachineInstr *MI) const {
   MCOptions.SanitizeAddress =
       MF->getFunction().hasFnAttribute(Attribute::SanitizeAddress);
 
+  // Emit warnings if we use reserved registers on the clobber list, as
+  // that might give surprising results.
+  std::vector<std::string> RestrRegs;
+  // Start with the first operand descriptor, and iterate over them.
+  for (unsigned I = InlineAsm::MIOp_FirstOperand, NumOps = MI->getNumOperands();
+       I < NumOps; ++I) {
+    const MachineOperand &MO = MI->getOperand(I);
+    if (MO.isImm()) {
+      unsigned Flags = MO.getImm();
+      if (InlineAsm::getKind(Flags) == InlineAsm::Kind_Clobber &&
+          MF->getRegInfo().isReserved(MI->getOperand(I + 1).getReg())) {
+        const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
+        RestrRegs.push_back(TRI->getName(MI->getOperand(I + 1).getReg()));
+      }
+      // Skip to one before the next operand descriptor, if it exists.
+      I += InlineAsm::getNumOperandRegisters(Flags);
+    }
+  }
+
+  if (!RestrRegs.empty()) {
+    unsigned BufNum = addInlineAsmDiagBuffer(OS.str(), LocMD);
+    auto &SrcMgr = DiagInfo->SrcMgr;
+    SMLoc Loc = SMLoc::getFromPointer(
+        SrcMgr.getMemoryBuffer(BufNum)->getBuffer().begin());
+
+    std::string Msg = "inline asm clobber list contains reserved registers: ";
+    for (auto I = RestrRegs.begin(), E = RestrRegs.end(); I != E; I++) {
+      if(I != RestrRegs.begin())
+        Msg += ", ";
+      Msg += *I;
+    }
+    std::string Note = "Reserved registers on the clobber list may not be "
+                "preserved across the asm statement, and clobbering them may "
+                "lead to undefined behaviour.";
+    SrcMgr.PrintMessage(Loc, SourceMgr::DK_Warning, Msg);
+    SrcMgr.PrintMessage(Loc, SourceMgr::DK_Note, Note);
+  }
+
   EmitInlineAsm(OS.str(), getSubtargetInfo(), MCOptions, LocMD,
                 MI->getInlineAsmDialect());
 

test/CodeGen/AArch64/inline-asm-clobber.ll

@@ -0,0 +1,8 @@
+; RUN: llc <%s -mtriple=aarch64-none-eabi 2>&1  | FileCheck %s
+
+; CHECK: warning: inline asm clobber list contains reserved registers: SP
+
+define void @foo() nounwind {
+  call void asm sideeffect "mov x7, #1", "~{x7},~{sp}"()
+  ret void
+}

test/CodeGen/ARM/inline-asm-clobber.ll

@@ -0,0 +1,27 @@
+; RUN: llc <%s -mtriple=arm-none-eabi 2>&1 | FileCheck %s -check-prefix=CHECK
+
+; RUN: llc <%s -mtriple=arm-none-eabi -relocation-model=rwpi 2>&1 \
+; RUN:   | FileCheck %s -check-prefix=RWPI
+
+; RUN: llc <%s -mtriple=arm-none-eabi --disable-fp-elim 2>&1 \
+; RUN:   | FileCheck %s -check-prefix=NO_FP_ELIM
+
+; CHECK: warning: inline asm clobber list contains reserved registers: SP, PC
+; CHECK: warning: inline asm clobber list contains reserved registers: R11
+; RWPI: warning: inline asm clobber list contains reserved registers: R9, SP, PC
+; RWPI: warning: inline asm clobber list contains reserved registers: R11
+; NO_FP_ELIM: warning: inline asm clobber list contains reserved registers: R11, SP, PC
+; NO_FP_ELIM: warning: inline asm clobber list contains reserved registers: R11
+
+define void @foo() nounwind {
+  call void asm sideeffect "mov r7, #1",
+    "~{r9},~{r11},~{r12},~{lr},~{sp},~{pc},~{r10}"()
+  ret void
+}
+
+define i32 @bar(i32 %i) {
+  %vla = alloca i32, i32 %i, align 4
+  tail call void asm sideeffect "mov r7, #1", "~{r11}"()
+  %1 = load volatile i32, i32* %vla, align 4
+  ret i32 %1
+}

test/CodeGen/X86/inline-asm-clobber.ll

@@ -0,0 +1,8 @@
+; RUN: llc <%s -mtriple=x86_64-unknown-unknown -- 2>&1  | FileCheck %s
+
+; CHECK: warning: inline asm clobber list contains reserved registers: RSP, EBP
+
+define void @foo() nounwind {
+  call void asm sideeffect "mov $$0x12, %eax", "~{eax},~{rsp},~{esi},~{ebp}"()
+  ret void
+}