1476011 - [Static Analysis] DEAD_STORE error in js/src/jit/x86-shared/Disassember-x86-shared.cpp

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, defect, P3)

Description

[Robert Bartlensky [:rbartlensky]]

js/src/jit/x86-shared/Disassembler-x86-shared.cpp:165: error: DEAD_STORE
  The value written to &opcode (type unsigned int) is never used.
  163.   {
  164.       VexOperandType type = VEX_PS;
  165. >     uint32_t opcode = OP_HLT;
  166.       uint8_t modrm = 0;
  167.       uint8_t sib = 0;

Comment 1

[Robert Bartlensky [:rbartlensky]]

Attachment: Bug 1476011: Initialize opcode on different value.

Review commit: https://reviewboard.mozilla.org/r/257254/diff/#index_header
See other reviews: https://reviewboard.mozilla.org/r/257254/

Comment 2

[Jan de Mooij [:jandem]]

Comment on attachment 8992389 [details]
Bug 1476011: Initialize opcode on different value.

https://reviewboard.mozilla.org/r/257254/#review264346

So the DEAD_STORE warning is only used when initializing to a non-zero value, and initializing to 0 is always considered fine? r=me if you're sure this is how it works.

Comment 3

[Robert Bartlensky [:rbartlensky]]

Comment on attachment 8992389 [details]
Bug 1476011: Initialize opcode on different value.

https://reviewboard.mozilla.org/r/257254/#review264346

DEAD_STORE warnings are concerned with not using the values that you assign to variables. Usually when you initialize a variable you either initialize it on 0 (which is fine from the POV of the analyzer), or on a non-zero value if you are going to use that value in some way later in the program.

For instance:

```
int x = getSomeNumber(); // fine because the value which is stored is also used
if (x == 5) {
  x = 2;
}
```

```
int x = getSomeNumber(); // DEAD_STORE -> you are storing a value in x, but never use it
                         // to fix the error you either drop the the return value of getSomeNumber, or
                         // use the return value before assigning another value to x
                         // basically DEAD_STORE = useless assingment, but it can also indicate that
                         // you forgot to use the value of x in between different assignments
x = getSomeOtherNumber();
```

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/c67bdec5c592
Initialize opcode on different value. r=jandem

Comment 5

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/c67bdec5c592