trans/codegen_c: Collapse Rust asm! double-dollar escape for GCC inline asm#412
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rautyrauty wants to merge 1 commit into
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trans/codegen_c: Collapse Rust asm! double-dollar escape for GCC inline asm#412rautyrauty wants to merge 1 commit into
rautyrauty wants to merge 1 commit into
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…g GCC inline asm
This is necessary because Rust asm! templates target LLVM's $-based
operand syntax (where $$ escapes a literal $), but mrustc lowers them
through GCC inline-asm where operands are %-prefixed and $ has no
escape. The doubled $ survives into the asm text; with .intel_syntax
noprefix GAS evaluates "$$0x80" as a non-8-bit expression and rejects
the int instruction, breaking rustix-1.0.8 on i586 ("int $$0x80" in
its i686 syscall wrappers).
thepowersgang
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May 24, 2026
Comment on lines
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| // Rust's `asm!` templates are passed verbatim to LLVM, where `$$` | ||
| // is the escape for a literal `$` (LLVM IR uses `$N` for operand | ||
| // references). The GCC inline-asm backend has no such escape, and | ||
| // a stray `$` survives into the GAS input as e.g. `int $0x80` — | ||
| // which GAS rejects in `.intel_syntax noprefix` mode with | ||
| // "operand type mismatch". Collapse `$$` here: drop it entirely | ||
| // for Intel-syntax blocks (so `int $$0x80` -> `int 0x80`), and | ||
| // halve it for AT&T blocks (so the immediate prefix survives). | ||
| auto xlat_dollars = [&](const std::string& s) { | ||
| std::string r; | ||
| r.reserve(s.size()); | ||
| for(size_t i = 0; i < s.size(); ++i) { | ||
| if(s[i] == '$' && i+1 < s.size() && s[i+1] == '$') { | ||
| if(se.options.att_syntax) r += '$'; | ||
| i++; | ||
| } | ||
| else { | ||
| r += s[i]; | ||
| } | ||
| } | ||
| return r; | ||
| }; |
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This translation should be in operator<<(::std::ostream& os, const FmtGccAsm& x) instead of as pre-processing pass.
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Rust
asm!templates target LLVM's$-based operand syntax (where$$escapes a literal$), but mrustc lowers them through the GCC inline-asm protocol where operands are%-prefixed and$has no escape. The doubled$therefore survives into the assembly text. With the surrounding.intel_syntax noprefixwrapper GAS evaluates"$$0x80"as a non-8-bit expression and rejects theintinstruction, breaking rustix-1.0.8 on i586 — its i686 syscall wrappers use"int $$0x80".Collapse
$$at codegen time: drop it for Intel-syntax blocks (so"int $$0x80"lowers to"int 0x80") and halve it for AT&T blocks (so the$-immediate prefix survives).