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<p class="MsoNormal">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:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> foo(struct s)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I instead find<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> foo(member1, member2, …)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">typedef struct S<o:p></o:p></p>
<p class="MsoNormal">{<o:p></o:p></p>
<p class="MsoNormal"> char a;<o:p></o:p></p>
<p class="MsoNormal"> char b;<o:p></o:p></p>
<p class="MsoNormal"> char c;<o:p></o:p></p>
<p class="MsoNormal">} S;<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Zachary Morgan<o:p></o:p></p>
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