Fix arm_neon.h to be clean under -fno-lax-vector-conversions.

[rsmith]

Summary

arm_neon.h produces a large number of errors under -fno-lax-vector-conversions,
which disables some deeply dangerous implicit conversions and which we really
need to make the default (PR17164).

This patch fixes the immediate problem by removing a bunch of incorrect casts
generated in the header. But it's only the first part of the problem: the other
part is fixing the incorrect expectations in the corresponding CodeGen test
files (in particular, in two ~20K line test files). However, I don't want to
touch that until I'm confident this change is correct.

[rsmith]

Remove unneeded test change.

[SjoerdMeijer]

Looks okay to me.

[rsmith]

This patch doesn't work, and the ARM NEON intrinsic header emitter is too opaque and mysterious for me to be able to fix this, and it's not even clear that the generator knows what the actual parameter types of the compiler builtins are. For now I think the best I can do is to leave ARM targets with lax vector conversions enabled, and disable it for all the other targets. :-(

(It's also completely ridiculous that we're generating a 63K LoC (2.1MB) header file for this; our approach to intrinsics headers is unreasonable in the extreme. But I digress.)

[SjoerdMeijer]

Yeah, sorry about that....

Do you perhaps have a test case or error that I can look at? Perhaps I or someone else here can help out a bit here.

[rsmith]

In D61717#1668980, @SjoerdMeijer wrote:

Do you perhaps have a test case or error that I can look at? Perhaps I or someone else here can help out a bit here.

You can reproduce the problem with, for example:

echo '#include <arm_neon.h>' | clang -target arm64-linux-gnu -arch +neon -fsyntax-only -x c - -fno-lax-vector-conversions

I've cleaned up all our other intrinsics headers to be clean under at least -flax-vector-conversions=integer. I'll be switching the default for all targets other than ARM NEON to that imminently. (It'll take a bit more work to be able to switch to -fno-lax-vector-conversions by default, but getting there should be our goal.)

I've filed PR43341 for this issue.

[SjoerdMeijer]

Many thanks! I've passed on the message, hopefully we can do something about this.

test/Headers/arm-neon-header.c

 // RUN: %clang_cc1 -triple thumbv7-apple-darwin10 -target-cpu cortex-a8 -fsyntax-only -Wvector-conversions -ffreestanding %s
 // RUN: %clang_cc1 -triple thumbv7-apple-darwin10 -target-cpu cortex-a8 -fsyntax-only -fno-lax-vector-conversions -ffreestanding %s
 // RUN: %clang_cc1 -x c++ -triple thumbv7-apple-darwin10 -target-cpu cortex-a8 -fsyntax-only -Wvector-conversions -ffreestanding %s
 
+// RUN: %clang_cc1 -triple arm64-linux-gnu -fallow-half-arguments-and-returns -target-feature +neon -target-feature +fullfp16 -ffreestanding -fsyntax-only %s -fno-lax-vector-conversions
+// RUN: %clang_cc1 -triple aarch64-linux-gnu -fallow-half-arguments-and-returns -target-feature +fullfp16 -target-feature +neon -ffreestanding -fsyntax-only %s -fno-lax-vector-conversions
+
 // RUN: %clang -fsyntax-only               -ffreestanding --target=aarch64-none-eabi -march=armv8.2-a+fp16 -std=c89 -xc %s
 // RUN: %clang -fsyntax-only -Wall -Werror -ffreestanding --target=aarch64-none-eabi -march=armv8.2-a+fp16 -std=c99 -xc %s
 // RUN: %clang -fsyntax-only -Wall -Werror -ffreestanding --target=aarch64-none-eabi -march=armv8.2-a+fp16 -std=c11 -xc %s
 
 // RUN: %clang -fsyntax-only               -ffreestanding --target=aarch64_be-none-eabi -march=armv8.2-a+fp16 -std=c89 -xc %s
 // RUN: %clang -fsyntax-only -Wall -Werror -ffreestanding --target=aarch64_be-none-eabi -march=armv8.2-a+fp16 -std=c99 -xc %s
 // RUN: %clang -fsyntax-only -Wall -Werror -ffreestanding --target=aarch64_be-none-eabi -march=armv8.2-a+fp16 -std=c11 -xc %s
 

utils/TableGen/NeonEmitter.cpp

if (hasSplat() && I == getSplatIdx()) {
       Arg += "}";
 
       CastToType = BaseType;
     } else {
       Arg = V.getName();
     }
 
     // Check if an explicit cast is needed.
-    if (CastToType.isVector()) {
+    if (CastToType.isVector() && (LocalCK == ClassB || CastToType.isChar())) {
       CastToType.makeInteger(8, true);
       Arg = "(" + CastToType.str() + ")" + Arg;
     }
 
     S += Arg + ", ";
   }
 
   // Extra constant integer to hold type class enum for this function, e.g. s8