[ASDAlexander77]

On x86-64 Linux, a call from tslang into C that passes an 8- or 16-bit integer or a bool could hand the C function a wrong value. tslang never emitted signext/zeroext; nothing under lib/TypeScript set either.

Found by #367's end-to-end bindgen test on Linux CI (run 36246544195):

expected: narrow -5 65535 -5 false true
got:      narrow 251 65535 -5 false true

fx_widen_s8(int8_t v) { return v; }, built by clang at -O2, returned 251 for -5.

Cause

On x86-64 SysV, clang-compiled code expects whoever hands over a small integer to have extended it to 32 bits:

• the caller, for an argument;
• the callee, for a result.

LLVM does that extension only when the parameter or result is marked signext/zeroext. tslang passed the bare i8, so the callee read -5's 8 bits as 251 in a 32-bit register. u16 65535 came out right only because its zero extension is the same number.

On Windows x64 the callee extends, so the bug never showed there.

Fix

FuncOpLowering (LowerToLLVM.cpp) now marks the parameters and result of a declaration, a function with no body:

• s8/s16: signext;
• u8/u16/boolean: zeroext.

A direct call takes the callee declaration's attributes, so the caller now extends: movl $-5, %edi for take_s8(-5), checked with --emit=asm --mtriple=x86_64-unknown-linux-gnu.

Not changed

• Definitions get no attributes. A signext parameter makes the callee trust its caller to have extended. tslang's indirect calls (closures, vtables, hybrid functions) carry no call-site attributes, so marking definitions would create new miscompiles in pure tslang code. So a tslang callback that C calls with a small integer, or whose small result C reads, is still unmarked.
• Indirect calls through a C function pointer carry no call-site attributes.
• Signless i8/i16 are left alone, since no type says which extension they need. tsbindgen emits sN/uN.

Tests

• test-compile-small-int-ext-*: eight IR checks on small-int-ext/declare.ts, covering s8, s16, u8, u16, bool, an s8 and a u16 result, and s32 left unmarked.
  • The seven small-type checks failed before the change; the s32 control passed both before and after.
• Full Release suite on Windows: 2892/2892 pass.
• CI on this PR: Windows and Linux green.
• End to end: tsbindgen v1: C headers to tslang bindings #367's branch with this commit cherry-picked, on Linux CI (run 36250596356): 2907/2907 pass. test-bindgen-plain and test-bindgen-namespace now pass, including the narrow row that failed without this fix.

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