[PATCH] D67160: [clang, ARM] Default to -fno-lax-vector-conversions in ARM v8.1-M.

[Simon Tatham via Phabricator via cfe-commits]

simon_tatham created this revision.
simon_tatham added reviewers: dmgreen, miyuki, ostannard.
Herald added subscribers: cfe-commits, kristof.beyls, javed.absar.
Herald added a project: clang.

The ACLE intrinsics for the MVE instruction set have polymorphic
variants: you can write vaddq(v,w) and have it automatically select
vaddq_s32, vaddq_u8, vaddq_f16, ... according to the types of the
vector variables v and w.

In order to implement this using __attribute__((overloadable)), I need
to turn on strict type-checking between the different vector types, or
else clang will believe that any vector type matches _all_ of those
function prototypes, and won't be able to select the right one.

Also, I think this makes sense for MVE anyway, on error-checking
grounds. For NEON, where every intrinsic in C source explicitly
describes the operation it's performing, you could argue that the
distinct vector types aren't essential; but in a context where getting
the wrong type can cause silent generation of the _wrong_ code, I
think it's better to warn users as soon as possible if they've written
code that isn't using vector types consistently.

So this commit introduces a (minimal) piece of infrastructure to allow
the default for OPT_flax_vector_conversions to vary based on the
target triple, and uses it to set the default specially for
ARMSubArch_v8_1m_mainline (in which the only possible vector
architecture will be MVE).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67160

Files:
  clang/lib/Driver/ToolChains/Clang.cpp


Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -1357,6 +1357,17 @@
   }
 }
 
+static bool isLaxVectorConversionsDefault(const llvm::Triple &Triple) {
+  switch (Triple.getArch()) {
+  default:
+    return true;
+
+  case llvm::Triple::thumb:
+  case llvm::Triple::thumbeb:
+    return Triple.getSubArch() != llvm::Triple::ARMSubArch_v8_1m_mainline;
+  }
+}
+
 static bool isNoCommonDefault(const llvm::Triple &Triple) {
   switch (Triple.getArch()) {
   default:
@@ -4674,10 +4685,13 @@
   if (TC.SupportsProfiling())
     Args.AddLastArg(CmdArgs, options::OPT_mfentry);
 
-  // -flax-vector-conversions is default.
-  if (!Args.hasFlag(options::OPT_flax_vector_conversions,
-                    options::OPT_fno_lax_vector_conversions))
+  if (const Arg *A = Args.getLastArg(options::OPT_flax_vector_conversions,
+                    options::OPT_fno_lax_vector_conversions)) {
+    if (A->getOption().matches(options::OPT_fno_lax_vector_conversions))
+      CmdArgs.push_back("-fno-lax-vector-conversions");
+  } else if (!isLaxVectorConversionsDefault(Triple)) {
     CmdArgs.push_back("-fno-lax-vector-conversions");
+  }
 
   if (Args.getLastArg(options::OPT_fapple_kext) ||
       (Args.hasArg(options::OPT_mkernel) && types::isCXX(InputType)))


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