ARM: fix VFP asm constraints

[jfb]

Summary

Rich Felker was sad that clang used 'w' and 'P' for VFP constraints when GCC documents them as 't' and 'w':

https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html

This was added way back in 2008:

http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20080421/005393.html

[jfb]

As a reference, here's the GCC source (instead of the docs): https://raw.githubusercontent.com/gcc-mirror/gcc/master/gcc/config/arm/constraints.md

[compnerd]

Test cases? It seems that 't' and 'w' are not available in thumb-1 mode.

[jfb]

• Add test.

[jfb]

Added a test.

Constraint validation is lacking in LLVM: it doesn't check that the input variable matches the constraint. I do pass mfpmath vfp to the test in case this gets fixed, but it's not strictly necessary right now.

The following is currently accepted:

int oops(int x) { // CHECK-LABEL: @oops(
  // CHECK: call float asm "vsqrt.f32 $0, $1", "=t,t"(float
  __asm__("vsqrt.f32 %0, %1"
          : "=t"(x)
          : "t"(x));
  return x;
}

Bitcode:

define arm_aapcscc i32 @oops(i32 %x) #1 {
  %1 = alloca i32, align 4
  store i32 %x, i32* %1, align 4
  %2 = load i32, i32* %1, align 4
  %3 = call i32 asm "vsqrt.f32 $0, $1", "=t,t"(i32 %2) #2, !srcloc !7
  store i32 %3, i32* %1, align 4
  %4 = load i32, i32* %1, align 4
  ret i32 %4
}

Here's a fun GCC comparison, which is also surprising in a different way.

Fixing this would be an entirely different patch.

[jfb]

Is this ready to land? I'm hoping to get it into the 3.8 branch since it affects musl libc's latest release.

[compnerd]

Yeah, constraint validation is not one of the highlights of the current implementation. This seems reasonable enough to merge I think.

cfe/trunk/lib/Basic/Targets.cpp

public:
   ArrayRef<const char *> getGCCRegNames() const override;
   ArrayRef<TargetInfo::GCCRegAlias> getGCCRegAliases() const override;
   bool validateAsmConstraint(const char *&Name,
                              TargetInfo::ConstraintInfo &Info) const override {
     switch (*Name) {
     default: break;
     case 'l': // r0-r7
     case 'h': // r8-r15
-    case 'w': // VFP Floating point register single precision
-    case 'P': // VFP Floating point register double precision
+    case 't': // VFP Floating point register single precision
+    case 'w': // VFP Floating point register double precision
       Info.setAllowsRegister();
       return true;
     case 'I':
     case 'J':
     case 'K':
     case 'L':
     case 'M':
       // FIXME