[cfe-dev] Direct Argument Passing in Clang
z-morgan at ti.com
Thu Aug 28 15:01:29 PDT 2014
I am in the process of adding a new ABI Info to lib/CodeGen/TargetInfo.cpp. When classifying arguments passing conventions, for small structs, we want to pass directly. That is, in classifyArgumentType(), we return ABIArgInfo::getDirect() in these cases. However, I noticed that the structure is then passed by its members individually. So where I would expect a call of the form:
I instead find
foo(member1, member2, ...)
This change in calling convention occurs in lib/CodeGen/CGCall.cpp - in each case near a comment stating that "If the coerce-to type is a first class aggregate, we flatten it" and further asserting that either way is semantically identical. I would presume this can only be true only if padding of structures is equivalent to padding and promotions of values on the stack. The test case I am looking at contains the use of the following as a parameter:
typedef struct S
As a whole, the struct will fit within at least 32 bits on the stack. When the members are separated, due to promotions, the struct will take up 3 * sizeof(int) on the stack.
I'm fairly certain that what I want to do is pass the arguments directly as stated above, but I wanted to check and see if there is a more preferred way for handling this.
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