diff options
Diffstat (limited to 'lib/Target/X86/README.txt')
| -rw-r--r-- | lib/Target/X86/README.txt | 39 |
1 files changed, 21 insertions, 18 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 9cb01a7..ccd15be 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -1352,14 +1352,15 @@ L7: L5: //===---------------------------------------------------------------------===// + Tail call optimization improvements: Tail call optimization currently -pushes all arguments on the top of the stack (their normal place if -that was a not tail call optimized functiong call ) before moving them -to actual stack slot. this is done to prevent overwriting of paramters -(see example below) that might be used, since the arguments of the -callee overwrites callers arguments. +pushes all arguments on the top of the stack (their normal place for +non-tail call optimized calls) before moving them to actual stack +slot. This is done to prevent overwriting of parameters (see example +below) that might be used, since the arguments of the callee +overwrites caller's arguments. - example: +example: int callee(int32, int64); int caller(int32 arg1, int32 arg2) { @@ -1371,39 +1372,41 @@ int caller(int32 arg1, int32 arg2) { [arg2] -> [(int64) [RETADDR] local ] -moving arg1 onto the stack slot of callee function would overwrite +Moving arg1 onto the stack slot of callee function would overwrite arg2 of the caller. Possible optimizations: - - only push those arguments to the top of the stack that are actual + - Only push those arguments to the top of the stack that are actual parameters of the caller function and have no local value in the - caller + caller. - in above example local does not need to be pushed onto the top of - the stack as it is definitetly not a caller's function parameter + In the above example local does not need to be pushed onto the top + of the stack as it is definitely not a caller's function + parameter. - - analyse the actual parameters of the callee to see which would - overwrite a caller paramter which is used by the callee and only - push them onto the top of the stack + - Analyse the actual parameters of the callee to see which would + overwrite a caller parameter which is used by the callee and only + push them onto the top of the stack. int callee (int32 arg1, int32 arg2); int caller (int32 arg1, int32 arg2) { return callee(arg1,arg2); } - here we don't need to write any variables to the top of the stack - since they don't overwrite each other + Here we don't need to write any variables to the top of the stack + since they don't overwrite each other. int callee (int32 arg1, int32 arg2); int caller (int32 arg1, int32 arg2) { return callee(arg2,arg1); } - here we need to push the arguments because they overwrite each other + Here we need to push the arguments because they overwrite each + other. - code for lowering directly onto callers arguments: + Code for lowering directly onto callers arguments: + SmallVector<std::pair<unsigned, SDOperand>, 8> RegsToPass; + SmallVector<SDOperand, 8> MemOpChains; + |