GCC: Difference between revisions

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==Useful flags==

* <tt>-fmax-errors=n</tt> print a maximum of n errors, 0 (the default) being unbounded

* <tt>-E</tt> to generate preprocessor output, <tt>-s</tt> to generate assembly output

* <tt>-fanalyzer</tt> (since 10.1) to do rich static analysis [https://clearlinux.org/blogs-news/major-improvements-gcc-101 ClearLinux blog]

==Optimization==

* Use '''-Q -Ox --help=optimizers''' to determine which optimization flags are enabled for a given -Ox setting (or read the info pages).

* [[Compiler Design|Dataflow analysis]] is only performed at -O2 or above, and thus at least this level of optimization is necessary for use-of-uninitialized-variable warnings and such.

==Extensions to [[C]]==

Extensions to the [[C]] language are documented at the [http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions online gcc docs], and in the Info pages distributed with gcc.

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* outputs, inputs and clobbers are expressed in a colon-delimited list of comma-delimited lists of the form [''asmsymbol''] "''constraints''" (''c symbol'')

** operand constraints are properties of the assembly code, not the values

** without "=" or "+" constraint modifier, operands are assumed to be read-only

** compiler verifies that all outputs are lvalues. types of operands are '''not''' checked.

** "cc" logical register ought be listed as a clobber if the conditional code register is changed

** "memory" must be listed as clobbered if memory is touched in an unpredictable fashion

* constraint modifiers:

** "=": operand is write-only (previous value needn't be preserved until write)

** "+": operand is read-write (can't be arbitrarily used)

** "&": operand is clobbered early (prior to use of all inputs), and thus can't be placed atop an input operand's

==Intermediate Representations==

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* "[http://www.cs.lth.se/home/Jonas_Skeppstedt/kongstad.pdf An Implementation of Global Value Numbering in the GNU Compiler Collection, with Performance Measurements]" (Kongstad 2004)

* [http://gcc.gnu.org/onlinedocs/gccint/ GNU Compiler Collection Internals]

* [[Core]] files

* [[Questions]]

@@ Line 1: / Line 1: @@
+==Useful flags==
+* <tt>-fmax-errors=n</tt> print a maximum of n errors, 0 (the default) being unbounded
+* <tt>-E</tt> to generate preprocessor output, <tt>-s</tt> to generate assembly output
+* <tt>-fanalyzer</tt> (since 10.1) to do rich static analysis [https://clearlinux.org/blogs-news/major-improvements-gcc-101 ClearLinux blog]
+==Optimization==
+* Use '''-Q -Ox --help=optimizers''' to determine which optimization flags are enabled for a given -Ox setting (or read the info pages).
+* [[Compiler Design|Dataflow analysis]] is only performed at -O2 or above, and thus at least this level of optimization is necessary for use-of-uninitialized-variable warnings and such.
 ==Extensions to [[C]]==
 Extensions to the [[C]] language are documented at the [http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions online gcc docs], and in the Info pages distributed with gcc.
@@ Line 29: / Line 37: @@
 * outputs, inputs and clobbers are expressed in a colon-delimited list of comma-delimited lists of the form [''asmsymbol''] "''constraints''" (''c symbol'')
 ** operand constraints are properties of the assembly code, not the values
+** without "=" or "+" constraint modifier, operands are assumed to be read-only
 ** compiler verifies that all outputs are lvalues. types of operands are '''not''' checked.
+** "cc" logical register ought be listed as a clobber if the conditional code register is changed
+** "memory" must be listed as clobbered if memory is touched in an unpredictable fashion
+* constraint modifiers:
+** "=": operand is write-only (previous value needn't be preserved until write)
+** "+": operand is read-write (can't be arbitrarily used)
+** "&": operand is clobbered early (prior to use of all inputs), and thus can't be placed atop an input operand's
 ==Intermediate Representations==
@@ Line 43: / Line 58: @@
 * "[http://www.cs.lth.se/home/Jonas_Skeppstedt/kongstad.pdf An Implementation of Global Value Numbering in the GNU Compiler Collection, with Performance Measurements]" (Kongstad 2004)
 * [http://gcc.gnu.org/onlinedocs/gccint/ GNU Compiler Collection Internals]
+* [[Core]] files
+* [[Questions]]

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