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sysv: fix a union passed as its last member when that member is narrower

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kalsprite
2026-08-13 22:51:07 -07:00
parent 2c947bb173
commit a2f92401d3
1 changed files with 14 additions and 19 deletions
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33
src/llvm_abi.cpp
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@@ -1041,21 +1041,15 @@ namespace lbAbiAmd64SysV {
return reg_classes;
}
// An SSE class that fills a whole eightbyte from offset 0, so nothing narrower
// starting at offset 0 can add to it.
gb_internal bool sse_class_covers_eightbyte(RegClass c) {
return c == RegClass_SSEDs || c == RegClass_SSEInt64;
}
// An SSE class positioned at offset 0 of its eightbyte. The `v` classes sit at
// offset 4 and are therefore DISJOINT from a 4-byte class at offset 0.
gb_internal bool sse_class_at_offset_zero(RegClass c) {
// How much of its eightbyte an SSE class occupies, which is what `llreg` turns
// it back into: the scalar classes are as wide as their element, and every `v`
// class becomes a vector spanning the whole eightbyte.
gb_internal i64 sse_class_width(RegClass c) {
switch (c) {
case RegClass_SSEHs: case RegClass_SSEFs: case RegClass_SSEDs:
case RegClass_SSEInt8: case RegClass_SSEInt16:
case RegClass_SSEInt32: case RegClass_SSEInt64:
return true;
case RegClass_SSEHs: return 2;
case RegClass_SSEFs: return 4;
}
return false;
return 8;
}
gb_internal void unify(Array<RegClass> *cls, i64 i, RegClass const newv) {
@@ -1091,12 +1085,13 @@ namespace lbAbiAmd64SysV {
case RegClass_SSEInt64:
return;
}
} else if (sse_class_covers_eightbyte(oldv) && sse_class_at_offset_zero(newv)) {
// The members OVERLAP -- a union. Last-writer-wins would pass
// `union{f64, f32}` as a 4-byte float and lose the top half. Restricted
// to a full-eightbyte old class against an offset-zero new one, because
// `struct{f32, f16}` is Fs then Hv at offset 4, which is disjoint and
// must still combine rather than pick.
} else if (is_sse(oldv) && is_sse(newv) && sse_class_width(oldv) > sse_class_width(newv)) {
// The members OVERLAP, a union. Last-writer-wins would pass
// `union{f64, f32}` as a 4-byte float, and `union{[2]f32, f32}` as one
// lane of two, losing the top half of the eightbyte either way. Keeping
// the WIDER class is what leaves `struct{f32, f16}` alone: that is Fs
// then Hv at offset 4, and Hv spans the eightbyte, so it still widens
// rather than gets picked over.
return;
}