[RISCV][clang] Enable atomics for 'Zalrsc', not just 'A'

'Zalrsc' only subtargets are atomics capable, ensure the RISCV target exposes
this correctly.

Author: slachowsky

clang/lib/Basic/Targets/RISCV.cpp

@@ -192,8 +192,11 @@ void RISCVTargetInfo::getTargetDefines(const LangOptions &Opts,
     Builder.defineMacro("__riscv_muldiv");
   }
 
-  if (ISAInfo->hasExtension("a")) {
+  // The "a" extension is composed of "zalrsc" and "zaamo"
+  if (ISAInfo->hasExtension("a"))
     Builder.defineMacro("__riscv_atomic");
+
+  if (ISAInfo->hasExtension("zalrsc")) {
     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1");
     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2");
     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4");

clang/lib/Basic/Targets/RISCV.h

@@ -195,7 +195,8 @@ class LLVM_LIBRARY_VISIBILITY RISCV32TargetInfo : public RISCVTargetInfo {
   void setMaxAtomicWidth() override {
     MaxAtomicPromoteWidth = 128;
 
-    if (ISAInfo->hasExtension("a"))
+    // "a" implies "zalrsc" which is sufficient to inline atomics
+    if (ISAInfo->hasExtension("zalrsc"))
       MaxAtomicInlineWidth = 32;
   }
 };
@@ -225,7 +226,8 @@ class LLVM_LIBRARY_VISIBILITY RISCV64TargetInfo : public RISCVTargetInfo {
   void setMaxAtomicWidth() override {
     MaxAtomicPromoteWidth = 128;
 
-    if (ISAInfo->hasExtension("a"))
+    // "a" implies "zalrsc" which is sufficient to inline atomics
+    if (ISAInfo->hasExtension("zalrsc"))
       MaxAtomicInlineWidth = 64;
   }
 };