diff options
Diffstat (limited to 'src/cmd/compile/internal-abi.md')
| -rw-r--r-- | src/cmd/compile/internal-abi.md | 24 |
1 files changed, 15 insertions, 9 deletions
diff --git a/src/cmd/compile/internal-abi.md b/src/cmd/compile/internal-abi.md index f4ef2cc869..b457f6ee74 100644 --- a/src/cmd/compile/internal-abi.md +++ b/src/cmd/compile/internal-abi.md @@ -153,6 +153,7 @@ Assigning a receiver, argument, or result V of underlying type T works as follows: 1. Remember I and FP. +1. If T has zero size, add T to the stack sequence S and return. 1. Try to register-assign V. 1. If step 2 failed, reset I and FP to the values from step 1, add T to the stack sequence S, and assign V to this field in S. @@ -295,6 +296,15 @@ An architecture may still define register meanings that aren’t compatible with ABI0, but these differences should be easy to account for in the compiler. +The assignment algorithm assigns zero-sized values to the stack +(assignment step 2) in order to support ABI0-equivalence. +While these values take no space themselves, they do result in +alignment padding on the stack in ABI0. +Without this step, the internal ABI would register-assign zero-sized +values even on architectures that provide no argument registers +because they don't consume any registers, and hence not add alignment +padding to the stack. + The algorithm reserves spill space for arguments in the caller’s frame so that the compiler can generate a stack growth path that spills into this reserved space. @@ -402,9 +412,6 @@ Special-purpose registers are as follows: | R15 | GOT reference temporary | Fixed if dynlink | | X15 | Zero value | Fixed | -TODO: We may start with the existing TLS-based g and move to R14 -later. - *Rationale*: These register meanings are compatible with Go’s stack-based calling convention except for R14 and X15, which will have to be restored on transitions from ABI0 code to ABIInternal code. @@ -455,13 +462,12 @@ The arithmetic status flags are treated like scratch registers and not preserved across calls. All other bits in RFLAGS are system flags. -The CPU is always in MMX technology state (not x87 mode). +At function calls and returns, the CPU is in x87 mode (not MMX +technology mode). -*Rationale*: Go on amd64 uses the XMM registers and never uses the x87 -registers, so it makes sense to assume the CPU is in MMX mode. -Otherwise, any function that used the XMM registers would have to -execute an EMMS instruction before calling another function or -returning (this is the case in the SysV ABI). +*Rationale*: Go on amd64 does not use either the x87 registers or MMX +registers. Hence, we follow the SysV platform conventions in order to +simplify transitions to and from the C ABI. At calls, the MXCSR control bits are always set as follows: |