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Do not generate allocations for zero sized allocations #63635

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merged 3 commits into from
Aug 18, 2019
Merged

Do not generate allocations for zero sized allocations #63635

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1fb854a
Do not generate allocations for zero sized allocations
oli-obk Aug 16, 2019
ab949fd
Cast only where necessary
oli-obk Aug 17, 2019
1ea88a8
Add tests
oli-obk Aug 17, 2019
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21 changes: 13 additions & 8 deletions src/librustc_codegen_llvm/common.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -333,14 +333,19 @@ impl ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
offset: Size,
) -> PlaceRef<'tcx, &'ll Value> {
assert_eq!(alloc.align, layout.align.abi);
let init = const_alloc_to_llvm(self, alloc);
let base_addr = self.static_addr_of(init, alloc.align, None);

let llval = unsafe { llvm::LLVMConstInBoundsGEP(
self.const_bitcast(base_addr, self.type_i8p()),
&self.const_usize(offset.bytes()),
1,
)};
let llval = if layout.size == Size::ZERO {
let llval = self.const_usize(alloc.align.bytes());
unsafe { llvm::LLVMConstIntToPtr(llval, self.type_ptr_to(self.type_i8p())) }
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Also this seems like you're creating a i8** instead of i8* (but ideally we'd just go directly to the final type).

} else {
let init = const_alloc_to_llvm(self, alloc);
let base_addr = self.static_addr_of(init, alloc.align, None);

unsafe { llvm::LLVMConstInBoundsGEP(
self.const_bitcast(base_addr, self.type_i8p()),
&self.const_usize(offset.bytes()),
1,
)}
};
let llval = self.const_bitcast(llval, self.type_ptr_to(layout.llvm_type(self)));
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Could you move the bitcast to the non-zero-len arm? And compute this final pointer type only once?

PlaceRef::new_sized(llval, layout, alloc.align)
}
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