[PATCH] Use of an unitialized register in 32-bit ARM template interpreter

[christoph.goettschkes at microdoc.com]

Hello,

while working with the OpenJDK 11 on a 32-bit ARMv7-A platform, I noticed
something weird in the template interpreter, regarding the template for
the bytecode instruction ldc2_w.

The type check for the operand is only done correctly if the ABI is a
hard-float one. For soft-float, the check is done wrong, using an
uninitialized register Rtemp. Please see the following diff:

diff -r 327d5994b2fb src/hotspot/cpu/arm/templateTable_arm.cpp
--- a/src/hotspot/cpu/arm/templateTable_arm.cpp	Tue Mar 12 11:13:39 2019 -0400
+++ b/src/hotspot/cpu/arm/templateTable_arm.cpp	Tue Jul 16 11:22:14 2019 +0200
@@ -515,36 +515,37 @@
 void TemplateTable::ldc2_w() {
   transition(vtos, vtos);
   const Register Rtags  = R2_tmp;
   const Register Rindex = R3_tmp;
   const Register Rcpool = R4_tmp;
   const Register Rbase  = R5_tmp;
 
   __ get_unsigned_2_byte_index_at_bcp(Rindex, 1);
 
   __ get_cpool_and_tags(Rcpool, Rtags);
   const int base_offset = ConstantPool::header_size() * wordSize;
   const int tags_offset = Array<u1>::base_offset_in_bytes();
 
   __ add(Rbase, Rcpool, AsmOperand(Rindex, lsl, LogBytesPerWord));
 
+  // get type from tags
+  __ add(Rtemp, Rtags, tags_offset);
+  __ ldrb(Rtemp, Address(Rtemp, Rindex));
+
   Label Condy, exit;
 #ifdef __ABI_HARD__
   Label Long;
-  // get type from tags
-  __ add(Rtemp, Rtags, tags_offset);
-  __ ldrb(Rtemp, Address(Rtemp, Rindex));
   __ cmp(Rtemp, JVM_CONSTANT_Double);
   __ b(Long, ne);
   __ ldr_double(D0_tos, Address(Rbase, base_offset));
 
   __ push(dtos);
   __ b(exit);
   __ bind(Long);
 #endif
 
   __ cmp(Rtemp, JVM_CONSTANT_Long);
   __ b(Condy, ne);
 #ifdef AARCH64
   __ ldr(R0_tos, Address(Rbase, base_offset));
 #else
   __ ldr(R0_tos_lo, Address(Rbase, base_offset + 0 * wordSize));
   
If the check for the type of the operand is done correctly, the call to
InterpreterRuntime::resolve_ldc should never happen. Currently, for 32-bit
soft-float arm, InterpreterRuntime::resolve_ldc is called if the operand 
for ldc2_w is of type long.

Also, I find it weird that the "condy_helper" code is genarted for the
ldc2_w bytecode instruction on 32-bit hard-float arm (and also on x86).
Aren't the only two valid types for ldc2_w long and double?

-- Christoph