MCParser: Error when .set reassigns a non-redefinable variable

The conditions in parseAssignmentExpression are conservative. We should
also report an error when a non-redefiniable variable (e.g. .equiv
followed by .set; .weakref followed by .set).

Make MCAsmStreamer::emitLabel call setOffset to make the behavior
similar to MCObjectStreamer. `isUndefined()` can now be replaced with
`isUnset()`.

Additionally, fix an AMDGPU API user (tested by a few tests including
MC/AMDGPU/hsa-v4.s)

Author: MaskRay

llvm/lib/MC/MCAsmStreamer.cpp

@@ -566,6 +566,11 @@ void MCAsmStreamer::emitELFSymverDirective(const MCSymbol *OriginalSym,
 
 void MCAsmStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
   MCStreamer::emitLabel(Symbol, Loc);
+  // FIXME: Fix CodeGen/AArch64/arm64ec-varargs.ll. emitLabel is followed by
+  // setVariableValue, leading to an assertion failure if setOffset(0) is
+  // called.
+  if (getContext().getObjectFileType() != MCContext::IsCOFF)
+    Symbol->setOffset(0);
 
   Symbol->print(OS, MAI);
   OS << MAI->getLabelSuffix();

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -6348,12 +6348,12 @@ bool parseAssignmentExpression(StringRef Name, bool allow_redef,
     //
     // FIXME: Diagnostics. Note the location of the definition as a label.
     // FIXME: Diagnose assignment to protected identifier (e.g., register name).
-    if (Sym->isUndefined() && !Sym->isUsed() && !Sym->isVariable())
+    if (!Sym->isUnset() && (!allow_redef || !Sym->isRedefinable()))
+      return Parser.Error(EqualLoc, "redefinition of '" + Name + "'");
+    else if (Sym->isUndefined() && !Sym->isUsed() && !Sym->isVariable())
       ; // Allow redefinitions of undefined symbols only used in directives.
     else if (Sym->isVariable() && !Sym->isUsed() && allow_redef)
       ; // Allow redefinitions of variables that haven't yet been used.
-    else if (!Sym->isUndefined() && (!Sym->isVariable() || !allow_redef))
-      return Parser.Error(EqualLoc, "redefinition of '" + Name + "'");
     else if (!Sym->isVariable())
       return Parser.Error(EqualLoc, "invalid assignment to '" + Name + "'");
     else if (!isa<MCConstantExpr>(Sym->getVariableValue()))

llvm/lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp

@@ -3080,6 +3080,7 @@ void AMDGPUAsmParser::initializeGprCountSymbol(RegisterKind RegKind) {
   assert(SymbolName && "initializing invalid register kind");
   MCSymbol *Sym = getContext().getOrCreateSymbol(*SymbolName);
   Sym->setVariableValue(MCConstantExpr::create(0, getContext()));
+  Sym->setRedefinable(true);
 }
 
 bool AMDGPUAsmParser::updateGprCountSymbols(RegisterKind RegKind,

llvm/test/MC/AsmParser/redef-err.s

@@ -0,0 +1,14 @@
+## Test redefinition errors. MCAsmStreamer::emitLabel is different from MCObjectStreamer. Test both streamers.
+# RUN: not llvm-mc -triple=x86_64 %s -o /dev/null 2>&1 | FileCheck %s --implicit-check-not=error:
+# RUN: not llvm-mc -filetype=obj -triple=x86_64 %s -o /dev/null 2>&1 | FileCheck %s --implicit-check-not=error:
+
+l:
+.set l, .
+# CHECK: [[#@LINE-1]]:9: error: redefinition of 'l'
+
+.equiv a, undef
+.set a, 3
+# CHECK: [[#@LINE-1]]:9: error: redefinition of 'a'
+
+.equiv a, undef
+# CHECK: [[#@LINE-1]]:11: error: redefinition of 'a'

llvm/test/MC/AsmParser/redef.s

@@ -0,0 +1,18 @@
+# RUN: not llvm-mc -filetype=obj -triple=x86_64 %s -o /dev/null 2>&1 | FileCheck %s --implicit-check-not=error:
+
+l:
+
+.data
+.set x, 0
+.long x
+x = .-.data
+.long x
+.set x,.-.data
+# CHECK: [[#@LINE-1]]:8: error: invalid reassignment of non-absolute variable 'x'
+## TODO This should be allowed
+.long x
+
+.globl l_v
+.set l_v, l
+.globl l_v
+.set l_v, l

llvm/test/MC/AsmParser/variables-invalid.s

@@ -1,5 +1,6 @@
 # RUN: llvm-mc -filetype=obj -triple=x86_64 %s | llvm-readelf -s - | FileCheck %s
 # RUN: not llvm-mc -filetype=obj -triple=x86_64 --defsym ERR=1 %s -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR --implicit-check-not=error:
+# RUN: not llvm-mc -filetype=obj -triple=x86_64 --defsym ERR2=1 %s -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR2 --implicit-check-not=error:
 
 // This is a long test that checks that the aliases created by weakref are
 // never in the symbol table and that the only case it causes a symbol to
@@ -104,11 +105,14 @@ alias:
 
 .weakref alias2, target
 .set alias2, 1
+# ERR: [[#@LINE-1]]:14: error: redefinition of 'alias2'
+.endif
 
+.ifdef ERR2
 .weakref cycle0, cycle1
 .weakref cycle1, cycle0
 call cycle0
-# ERR: <unknown>:0: error: cyclic dependency detected for symbol 'cycle0'
-# ERR: [[#@LINE-2]]:1: error: expected relocatable expression
+# ERR2: <unknown>:0: error: cyclic dependency detected for symbol 'cycle0'
+# ERR2: [[#@LINE-2]]:1: error: expected relocatable expression
 
 .endif