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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* <tt>aligned</tt> - takes a single parameter, the minimum number of bytes at which to align the function. <tt>-falign-functions</tt> will override this, if larger.

* <tt>malloc</tt> - indicates that any value returned does not [[Compiler Design|alias]] any other currently-valid pointers

* <tt>alloc_size</tt> - takes one or two parameters and indicates that the function will return a pointer to an allocated chunk of memory having either the size provided by a single argument, or the product of two arguments. This is necessary for ~~the~~ <tt>__builtin_object_size</tt>'s correct functioning.

* <tt>alloc_size</tt> - takes one or two parameters and indicates that the function will return a pointer to an allocated chunk of memory having either the size provided by a single argument, or the product of two arguments. This is necessary for <tt>__builtin_object_size</tt>'s correct functioning.

* <tt>pure</tt> (2.96) - indicates that the function has no side-effects save its return value, which is based only on calls to other <tt>pure</tt> functions, the function's own parameters and/or non-volatile global memory.

* <tt>const</tt> (2.5) - stronger than <tt>pure</tt>, a <tt>pure</tt> function which does not dereference any pointer parameters, use global memory or call non-<tt>const</tt> functions.

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===Inline Assembly===

* There's no need to use inline assembly for [[SIMD]]; use [http://gcc.gnu.org/onlinedocs/gcc/Target-Builtins.html#Target-Builtins Target-Specific Builtins] and [http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html#Vector-Extensions Vector Extensions]

* There's no need to use inline assembly for [[SIMD]]; use [http://gcc.gnu.org/onlinedocs/gcc/Target-Builtins.html#Target-Builtins Target-Specific Builtins] and [http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html#Vector-Extensions Vector Extensions], or [http://gcc.gnu.org/projects/tree-ssa/vectorization.html autovectorization] if applicable.

* Functions only referenced by inline assembly might not have code generated for them; use of the <tt>used</tt> [[GCC#Function_Attributes|function attribute]] will force generation.

* The [http://sourceware.org/binutils/docs/as/index.html GNU assembler] (gas) is used for assembly and syntax

* The [http://sourceware.org/binutils/docs/as/index.html GNU assembler] (gas) is used for assembly and syntax.

* Statements can be arbitrarily reordered by default, or anchored with the <tt>volatile</tt> qualifier

* 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==

* [http://gcc.gnu.org/onlinedocs/gccint/~~index~~.html~~#toc_RTL~~ RTL]: The Register Transfer Language, GCC's older IR (still used for late optimization passes)

* [http://gcc.gnu.org/onlinedocs/gccint/RTL.html RTL]: The Register Transfer Language, GCC's older IR (still used for late optimization passes)

* [http://gcc.gnu.org/onlinedocs/gccint/GENERIC.html#GENERIC GENERIC]: A loose IR to which frontends must now compile

* [http://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html#GIMPLE GIMPLE]: A restricted subset of GENERIC, on which most optimizations are performed

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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 12: / Line 20: @@
 * <tt>aligned</tt> - takes a single parameter, the minimum number of bytes at which to align the function. <tt>-falign-functions</tt> will override this, if larger.
 * <tt>malloc</tt> - indicates that any value returned does not [[Compiler Design|alias]] any other currently-valid pointers
-* <tt>alloc_size</tt> - takes one or two parameters and indicates that the function will return a pointer to an allocated chunk of memory having either the size provided by a single argument, or the product of two arguments. This is necessary for the <tt>__builtin_object_size</tt>'s correct functioning.
+* <tt>alloc_size</tt> - takes one or two parameters and indicates that the function will return a pointer to an allocated chunk of memory having either the size provided by a single argument, or the product of two arguments. This is necessary for <tt>__builtin_object_size</tt>'s correct functioning.
 * <tt>pure</tt> (2.96) - indicates that the function has no side-effects save its return value, which is based only on calls to other <tt>pure</tt> functions, the function's own parameters and/or non-volatile global memory.
 * <tt>const</tt> (2.5) - stronger than <tt>pure</tt>, a <tt>pure</tt> function which does not dereference any pointer parameters, use global memory or call non-<tt>const</tt> functions.
@@ Line 23: / Line 31: @@
 ===Inline Assembly===
-* There's no need to use inline assembly for [[SIMD]]; use [http://gcc.gnu.org/onlinedocs/gcc/Target-Builtins.html#Target-Builtins Target-Specific Builtins] and [http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html#Vector-Extensions Vector Extensions]
+* There's no need to use inline assembly for [[SIMD]]; use [http://gcc.gnu.org/onlinedocs/gcc/Target-Builtins.html#Target-Builtins Target-Specific Builtins] and [http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html#Vector-Extensions Vector Extensions], or [http://gcc.gnu.org/projects/tree-ssa/vectorization.html autovectorization] if applicable.
 * Functions only referenced by inline assembly might not have code generated for them; use of the <tt>used</tt> [[GCC#Function_Attributes|function attribute]] will force generation.
-* The [http://sourceware.org/binutils/docs/as/index.html GNU assembler] (gas) is used for assembly and syntax
+* The [http://sourceware.org/binutils/docs/as/index.html GNU assembler] (gas) is used for assembly and syntax.
+* Statements can be arbitrarily reordered by default, or anchored with the <tt>volatile</tt> qualifier
+* 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==
-* [http://gcc.gnu.org/onlinedocs/gccint/index.html#toc_RTL RTL]: The Register Transfer Language, GCC's older IR (still used for late optimization passes)
+* [http://gcc.gnu.org/onlinedocs/gccint/RTL.html RTL]: The Register Transfer Language, GCC's older IR (still used for late optimization passes)
 * [http://gcc.gnu.org/onlinedocs/gccint/GENERIC.html#GENERIC GENERIC]: A loose IR to which frontends must now compile
 * [http://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html#GIMPLE GIMPLE]: A restricted subset of GENERIC, on which most optimizations are performed
@@ Line 39: / 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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