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Diffstat (limited to 'doc/abi.txt')
| -rw-r--r-- | doc/abi.txt | 159 |
1 files changed, 82 insertions, 77 deletions
diff --git a/doc/abi.txt b/doc/abi.txt index 976c827..71ff263 100644 --- a/doc/abi.txt +++ b/doc/abi.txt @@ -1,3 +1,4 @@ + ================ System V ABI x64 ================ @@ -11,72 +12,75 @@ the IR description document for more information about them. -:|: ABI Subset Implemented +- ABI Subset Implemented +------------------------ Data classes of interest as defined by the ABI: - - INTEGER - - SSE - - MEMORY - - -Classification: - -1. The size of each argument gets rounded up to eightbytes. - (It keeps the stack always 8 bytes aligned.) -2. _Bool, char, short, int, long, long long and pointers - are in the INTEGER class. In the context of QBE, it - means that 'l' and 'w' are in the INTEGER class. -3. float and double are in the SSE class. In the context - of QBE, it means that 's' and 'd' are in the SSE class. -4. If the size of an object is larger than two eightbytes - or if contains unaligned fields, it has class MEMORY. - In the context of QBE, those are big aggregate types - and "dark" types. -5. Otherwise, recursively classify fields and determine - the class of the two eightbytes using the classes of - their components. If any is INTEGER the result is - INTEGER, otherwise the result is SSE. - -Passing: - - - Classify arguments in order. - - INTEGER arguments use in order %rdi %rsi %rdx %rcx - %r8 %r9. - - SSE arguments use in order %xmm0 - %xmm7. - - MEMORY gets passed on the stack. They are "pushed" - in the right-to-left order, so from the callee's - point of view, the left-most argument appears first - on the stack. - - When we run out of registers for an aggregate, revert - the assignment for the first eightbytes and pass it - on the stack. - - When all registers are taken, write arguments on the - stack from right to left. - - When calling a variadic function, %al stores the number - of vector registers used to pass arguments (it must be - an upper bound and does not have to be exact). - - Registers %rbx, %r12 - %r15 are callee-save. - -Returning: - - - Classify the return type. - - Use %rax and %rdx in order for INTEGER return values. - - Use %xmm0 and %xmm1 in order for SSE return values. - - I the return value's class is MEMORY, the first - argument of the function %rdi was a pointer to an - area big enough to fit the return value. The function - writes the return value there and returns the address - (that was in %rdi) in %rax. - - -:|: Alignment on the Stack + * INTEGER + * SSE + * MEMORY + + +~ Classification + + 1. The size of each argument gets rounded up to eightbytes. + (It keeps the stack always 8 bytes aligned.) + 2. _Bool, char, short, int, long, long long and pointers + are in the INTEGER class. In the context of QBE, it + means that 'l' and 'w' are in the INTEGER class. + 3. float and double are in the SSE class. In the context + of QBE, it means that 's' and 'd' are in the SSE class. + 4. If the size of an object is larger than two eightbytes + or if contains unaligned fields, it has class MEMORY. + In the context of QBE, those are big aggregate types + and "dark" types. + 5. Otherwise, recursively classify fields and determine + the class of the two eightbytes using the classes of + their components. If any is INTEGER the result is + INTEGER, otherwise the result is SSE. + +~ Passing + + * Classify arguments in order. + * INTEGER arguments use in order `%rdi` `%rsi` `%rdx` + `%rcx` `%r8` `%r9`. + * SSE arguments use in order `%xmm0` - `%xmm7`. + * MEMORY gets passed on the stack. They are "pushed" + in the right-to-left order, so from the callee's + point of view, the left-most argument appears first + on the stack. + * When we run out of registers for an aggregate, revert + the assignment for the first eightbytes and pass it + on the stack. + * When all registers are taken, write arguments on the + stack from right to left. + * When calling a variadic function, %al stores the number + of vector registers used to pass arguments (it must be + an upper bound and does not have to be exact). + * Registers `%rbx`, `%r12` - `%r15` are callee-save. + +~ Returning + + * Classify the return type. + * Use `%rax` and `%rdx` in order for INTEGER return + values. + * Use `%xmm0` and `%xmm1` in order for SSE return values. + * I the return value's class is MEMORY, the first + argument of the function `%rdi` was a pointer to an + area big enough to fit the return value. The function + writes the return value there and returns the address + (that was in `%rdi`) in `%rax`. + + +- Alignment on the Stack +------------------------ The ABI is unclear on the alignment requirement of the stack. What must be ensured is that, right before executing a 'call' instruction, the stack pointer %rsp is aligned on 16 bytes. On entry of the called function, the stack pointer is 8 modulo 16. Since most -functions will have a prelude pushing %rbp, the frame +functions will have a prelude pushing `%rbp`, the frame pointer, upon entry of the body code of the function is also aligned on 16 bytes (== 0 mod 16). @@ -85,7 +89,7 @@ g() to f(). | | | g() locals | - +-------------+ + +-------------+ ^ | | \ | | stack arg 2 | ' | |xxxxxxxxxxxxx| | f()'s MEMORY @@ -102,26 +106,27 @@ g() to f(). -> %rsp Legend: - - xxxxx Optional padding. - + * `xxxxx` Optional padding. + -:|: Remarks +- Remarks +--------- - - A struct can be returned in registers in one of three - ways. Either %rax, %rdx are used, or %xmm0, %xmm1, - or finally %rax, %xmm0. This should be clear from - the "Returning" section above. + * A struct can be returned in registers in one of three + ways. Either `%rax`, `%rdx` are used, or `%xmm0`, + `%xmm1`, or finally `%rax`, `%xmm0`. This should be + clear from the <@Returning> section above. - - The size of the arguments area of the stack needs to - be computed first, then arguments are packed starting - from the bottom of the argument area, respecting - alignment constraints. The ABI mentions "pushing" - arguments in right-to-left order, but I think it's a - mistaken view because of the alignment constraints. + * The size of the arguments area of the stack needs to + be computed first, then arguments are packed starting + from the bottom of the argument area, respecting + alignment constraints. The ABI mentions "pushing" + arguments in right-to-left order, but I think it's a + mistaken view because of the alignment constraints. - Example: If three 8 bytes MEMORY arguments are passed - to the callee and the caller's stack pointer is 16 bytes - algined, the layout will be like this. + Example: If three 8 bytes MEMORY arguments are passed + to the callee and the caller's stack pointer is 16 bytes + algined, the layout will be like this. +-------------+ |xxxxxxxxxxxxx| padding @@ -130,5 +135,5 @@ Legend: | stack arg 1 | +-------------+ -> 0 mod 16 - The padding must not be at the end of the stack area. - A "pushing" logic would put it at the end. + The padding must not be at the end of the stack area. + A "pushing" logic would put it at the end. |