| Summary: | .intel_syntax misassembles a forward-referenced .equ as a data reference | ||
|---|---|---|---|
| Product: | binutils | Reporter: | Josh Triplett <josh> |
| Component: | gas | Assignee: | unassigned |
| Status: | RESOLVED WONTFIX | ||
| Severity: | normal | CC: | bug-binutils |
| Priority: | P2 | ||
| Version: | 2.20 | ||
| Target Milestone: | --- | ||
| Host: | x86_64-linux-gnu | Target: | x86_64-linux-gnu |
| Build: | x86_64-linux-gnu | Last reconfirmed: | |
I originally observed this bug in in 16-bit assembly with .code16, but as shown in this report I can also reproduce it in 64-bit assembly. When assembler sees
mov ax, AN_EQU
AN_EQU is undefined and treats it as a symbol. Later AN_EQU is
resolved to 5. For
mov $AN_EQU, %ax
assembler knows the operand is an immediate due to `$'. There
is not much assembler can do here.
If the assembler knows enough to go back and substitute the value afterward, it knows that it emitted a data reference when it should have emitted an immediate. It could at least spit out an error at that point rather than assembling incorrect code; that would have saved me an hour of debugging and disassembly. Also, the assembler seems to support forward references in various other places; why not for equates? |
The following three-line text case assembles incorrectly: .intel_syntax noprefix mov ax, AN_EQU .equ AN_EQU, 5 Compiled with "gcc -c test.S" Disassembled with "objdump -M intel -d test.o": 0: 66 8b 04 25 05 00 00 mov ax,WORD PTR ds:0x5 7: 00 For some reason, this assembled as a memory dereference of ds:0x5. Somehow, gas knew enough to substitute the value 5, but didn't know to treat it as an immediate. If I move the equate before the instruction, it assembles correctly: .intel_syntax noprefix .equ AN_EQU, 5 mov ax, AN_EQU 0: 66 b8 05 00 mov ax,0x5 If I substitute a literal 5 in the instruction, it assembles correctly: .intel_syntax noprefix mov ax, 5 0: 66 b8 05 00 mov ax,0x5 And if I use AT&T syntax, it assembles correctly: .att_syntax mov $AN_EQU, %ax .equ AN_EQU, 5 0: 66 b8 05 00 mov ax,0x5