Make __CHERI__ a synonym for __CHERI_PURE_CAPABILITY__

WARNING: This is a breaking change for hybrid + standard C/C++ code.

Rationale: We strongly encourage all CHERI consumers to use a
pure-capability ABI whenever possible. Therefore it should be the
easiest thing to type and less preferred options should be longer. The
existing macros date to the earliest compiler support which barely
supported capability types. Before broad adoption of CHERI we should
make the macros be sensible. We're past Stuart Feldman's too many users
of make to change the tabs threshold (12), but few codebases will be
effected, the required changes are simple can be safely performed
without automatic review, and we shouldn't have to live with this aspect
of history forever.

Backwards compatible code transition:

For purecap + standard C/C++ code (no hybrid):
  No change required.

For hybrid + standard C/C++ (with or without purecap):
  defined(__CHERI__) -> __has_feature(capabilities)
or
  defined(__CHERI__) -> defined(__CHERI__) || defined(__CHERI_HYBRID__)

Once the need for compatability with old compilers is passed, further
simplication is possible.

For purecap + standard C/C++ code:
  __CHERI_PURE_CAPABILITY__ -> __CHERI__

For hybrid + standard C/C++ code:
  __has_feature(capabilities) && !defined(__CHERI_PURE_CAPABILITY__) ->
      defined(__CHERI_HYBRID__)

Author: brooksdavis

clang/lib/Frontend/InitPreprocessor.cpp

@@ -1122,7 +1122,6 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
   if (TI.SupportsCapabilities()) {
     const uint64_t CapWidth = TI.getCHERICapabilityWidth();
     const uint64_t CapRange = TI.getPointerRangeForCHERICapability();
-    Builder.defineMacro("__CHERI__", "1"); // TODO: or define __CHERI__ to 128/256?
     Builder.defineMacro("__CHERI_CAPABILITY_WIDTH__", Twine(CapWidth));
     DefineTypeSizeof("__SIZEOF_CHERI_CAPABILITY__", CapWidth, TI, Builder);
     Builder.defineMacro("__CHERI_ADDRESS_BITS__", Twine(CapRange));
@@ -1137,6 +1136,8 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
     DefineTypeSize("__UINTCAP_MAX__", TI.getIntTypeByWidth(CapRange, false), TI, Builder);
 
     if (TI.areAllPointersCapabilities()) {
+      Builder.defineMacro("__CHERI__");
+
       // XXXAR is there a reason we use two instead of just defining it?
       // I don't think we have any checks that rely on the value
       Builder.defineMacro("__CHERI_PURE_CAPABILITY__", "2");

clang/test/Driver/cheri/bounds-mode.c

@@ -17,3 +17,4 @@
 // CHECK: #define __CHERI_PURE_CAPABILITY__ 2
 // SUBOBJECT-DISABLED-NOT: __CHERI_SUBOBJECT_BOUNDS__
 // SUBOBJECT-SAFE: #define __CHERI_SUBOBJECT_BOUNDS__ 2
+// CHECK: #define __CHERI__ 1

clang/test/Preprocessor/cheri-cap-sizes.c

@@ -37,8 +37,9 @@
 // CHECK: #define __CHERI_ADDRESS_BITS__ 64
 // CHERI128: #define __CHERI_CAPABILITY_WIDTH__ 128
 // PURECAP: #define __CHERI_PURE_CAPABILITY__ 2
+// PURECAP: #define __CHERI__ 1
 // MIPS-NOT: __CHERI_PURE_CAPABILITY__
-// CHECK: #define __CHERI__ 1
+// MIPS-NOT: __CHERI__
 // MIPS: #define __CHERI_HYBRID__ 1
 // Note: 64-bit range for intcap makes more sense than the full range for pointers
 // CHECK: #define __INTCAP_MAX__ 9223372036854775807L

clang/test/Preprocessor/cheri-features.c

@@ -25,10 +25,10 @@ void* __capability x = 0;
 
 #if defined(__CHERI__)
 #pragma message("__CHERI__ defined")
-// CHECK-CHERI: warning: __CHERI__ defined
 void* __capability y = 0;
 #else
 #pragma message("__CHERI__ not defined")
+// CHECK-CHERI: warning: __CHERI__ not defined
 // CHECK-MIPS: warning: __CHERI__ not defined
 #endif
 

clang/test/Preprocessor/cheri-riscv-feature-flags.c

@@ -18,7 +18,6 @@
 // CHECK: #define __CHERI_CAP_PERMISSION_PERMIT_STORE_LOCAL__ 64
 // CHECK: #define __CHERI_CAP_PERMISSION_PERMIT_STORE__ 8
 // CHECK: #define __CHERI_CAP_PERMISSION_PERMIT_UNSEAL__ 512
-// CHECK: #define __CHERI__ 1
 // CHECK: #define __CHERI_HYBRID__ 1
 // CHECK32: #define __SIZEOF_CHERI_CAPABILITY__ 8
 // CHECK64: #define __SIZEOF_CHERI_CAPABILITY__ 16

clang/test/Preprocessor/init.c

@@ -434,7 +434,6 @@
 // CHERI-COMMON-NEXT: #define __CHERI_CAP_PERMISSION_PERMIT_STORE_LOCAL__ 64
 // CHERI-COMMON-NEXT: #define __CHERI_CAP_PERMISSION_PERMIT_STORE__ 8
 // CHERI-COMMON-NEXT: #define __CHERI_CAP_PERMISSION_PERMIT_UNSEAL__ 512
-// CHERI-COMMON-NEXT: #define __CHERI__ 1
 // CHERI-COMMON-NEXT: #define __CHERI_HYBRID__ 1
 // CHERI64:           #define __POINTER_WIDTH__ 32
 // CHERI128:          #define __POINTER_WIDTH__ 64
@@ -487,6 +486,7 @@
 // CHERI128-PURECAP: #define _MIPS_FPSET 32
 // CHERI128-PURECAP: #define __CHERI_PURE_CAPABILITY__ 2
 // CHERI128-PURECAP: #define __CHERI_SANDBOX__ 4
+// CHERI128-PURECAP: #define __CHERI__ 1
 // CHERI128-PURECAP: #define __INTPTR_FMTd__ "Pd"
 // CHERI128-PURECAP: #define __INTPTR_FMTi__ "Pi"
 // CHERI128-PURECAP: #define __INTPTR_MAX__ 9223372036854775807L