ARM64: Fix and improve MacroAssembler::Printf.

src/arm64/instructions-arm64.h

Index: src/arm64/instructions-arm64.h
diff --git a/src/arm64/instructions-arm64.h b/src/arm64/instructions-arm64.h
index 968ddace0468ffb330130c59e02a52662aa34b6f..81ef6ad3f883502f804eaa1c0ff7375b786e1ec7 100644
--- a/src/arm64/instructions-arm64.h
+++ b/src/arm64/instructions-arm64.h
@@ -416,24 +416,38 @@ const Instr kImmExceptionIsUnreachable = 0xdebf;
 // A pseudo 'printf' instruction. The arguments will be passed to the platform
 // printf method.
 const Instr kImmExceptionIsPrintf = 0xdeb1;
-// Parameters are stored in ARM64 registers as if the printf pseudo-instruction
-// was a call to the real printf method:
-//
-// x0: The format string, then either of:
+// Most parameters are stored in ARM64 registers as if the printf
+// pseudo-instruction was a call to the real printf method:
+//      x0: The format string.
 //   x1-x7: Optional arguments.
 //   d0-d7: Optional arguments.
 //
-// Floating-point and integer arguments are passed in separate sets of
-// registers in AAPCS64 (even for varargs functions), so it is not possible to
-// determine the type of location of each arguments without some information
-// about the values that were passed in. This information could be retrieved
-// from the printf format string, but the format string is not trivial to
-// parse so we encode the relevant information with the HLT instruction.
-// - Type
-//    Either kRegister or kFPRegister, but stored as a uint32_t because there's
-//    no way to guarantee the size of the CPURegister::RegisterType enum.
-const unsigned kPrintfTypeOffset = 1 * kInstructionSize;
-const unsigned kPrintfLength = 2 * kInstructionSize;
+// Also, the argument layout is described inline in the instructions:
+//  - arg_count: The number of arguments.
+//  - arg_pattern: A set of PrintfArgPattern values, packed into two-bit fields.
+//
+// Floating-point and integer arguments are passed in separate sets of registers
+// in AAPCS64 (even for varargs functions), so it is not possible to determine
+// the type of each argument without some information about the values that were
+// passed in. This information could be retrieved from the printf format string,
+// but the format string is not trivial to parse so we encode the relevant
+// information with the HLT instruction.
+const unsigned kPrintfArgCountOffset = 1 * kInstructionSize;
+const unsigned kPrintfArgPatternListOffset = 2 * kInstructionSize;
+const unsigned kPrintfLength = 3 * kInstructionSize;
+
+const unsigned kPrintfMaxArgCount = 4;
+
+// The argument pattern is a set of two-bit-fields, each with one of the
+// following values:
+enum PrintfArgPattern {
+  kPrintfArgW = 1,
+  kPrintfArgX = 2,
+  // There is no kPrintfArgS because floats are always converted to doubles in C
+  // varargs calls.
+  kPrintfArgD = 3
+};
+static const unsigned kPrintfArgPatternBits = 2;
 
 // A pseudo 'debug' instruction.
 const Instr kImmExceptionIsDebug = 0xdeb0;