0000117: Detect and optimize noreturn calls in optimize_funccalls

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Last Update

0000117

[libFirm] optimisations

minor

N/A

2013-09-03 23:30

2014-09-10 12:51

Reporter

Matze

View Status

public

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Priority

normal

Resolution

open

Status

confirmed

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Summary

0000117: Detect and optimize noreturn calls in optimize_funccalls

Description

noreturn calls can be found a lot in error handling paths which often end in abort() or similar. In many projects they are hidden in wrapper functions which first report an error before calling abort().

Infering noreturn should be easy (just look if there are any return statement in the function). We should then be able to eliminate some control flow paths from the executables.

Example:
void panic(const char *msg) { fprintf(stderr, "%s\n", msg); abort(); }

void normal_code(void) {
// ...
if (...) { panic("foo"); }
}

Here we could infer that panic() will never return (because abort is noreturn). This can be used to guide optimisations:
* noreturn functions do not need to be inlined (decrease code size)
* noreturn functions remove paths from the control flow graph which can trigger other optimisations. This needs actual support in form as currently noreturn is only handlded by the cparser frontend when constructing such calls, we should have a phase in localopt, opt_funccall or maybe optimize_control_flow that removes code (esp. jumps) after noreturn calls.

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Notes
(0000221) Matze (administrator) 2014-09-10 12:51	Detection code has been implemented in f5733374171db4987b27839784bc5a3cfd3174e3 but we still lack a transformation

Issue History
Date Modified	Username	Field	Change
2013-09-03 23:30	Matze	New Issue
2013-09-03 23:31	Matze	Status	new => confirmed
2013-09-19 23:21	Matze	Description Updated
2013-09-19 23:27	Matze	Description Updated
2014-09-10 12:51	Matze	Note Added: 0000221

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