246:
The two families of extensions represent different understandings of division of labor in processing inline assembly. The GCC form preserves the overall syntax of the language and compartmentizes what the compiler needs to know: what is needed and what is changed. It does not explicitly require the
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On the other hand, inline assembler poses a direct problem for the compiler itself as it complicates the analysis of what is done to each variable, a key part of register allocation. This means the performance might actually decrease. Inline assembler also complicates future porting and maintenance
270:
The Rust language has since migrated to a syntax abstracting away inline assembly options further than the LLVM (GCC-style) version. It provides enough information to allow transforming the block into an externally-assembled function if the backend could not handle embedded assembly.
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provides ease of writing, but it requires the compiler itself to know about opcode names and their clobbering properties, demanding extra attention in maintenance and porting. It is still possible to check GCC-style assembly for clobber mistakes with knowledge of the instruction set.
238:(MSVC), Borland/Embarcadero C compiler, and descendants. This syntax is not based on ISO rules at all; programmers simply write ASM inside a block without needing to conform to C syntax. Variables are available as if they are registers and some C expressions are allowed.
228:. GCC uses a direct extension of the ISO rules: assembly code template is written in strings, with inputs, outputs, and clobbered registers specified after the strings in colons. C variables are used directly while register names are quoted as string literals.
209:
In practical use, inline assembly operating on values is rarely standalone as free-floating code. Since the programmer cannot predict what register a variable is assigned to, compilers typically provide a way to substitute them in as an extension.
303:. Such calls are normally written with the aid of macros; the full code is included for clarity. In this particular case, the wrapper performs a system call of a number given by the caller with three operands, returning the result.
267:
uses a DSL similar to the MSVC extension officially for x86_64, but the LLVM-based LDC also provides the GCC-style syntax on every architecture. MSVC only supports inline assembler on 32-bit x86.
284:
Calling an operating system directly is generally not possible under a system using protected memory. The OS runs at a more privileged level (kernel mode) than the user (user mode); a (software)
201:
This definition, however, is rarely used in actual C, as it is simultaneously too liberal (in the interpretation) and too restricted (in the use of one string literal only).
141:
and interrupts: High-level languages rarely have a direct facility to make arbitrary system calls, so assembly code is used. Direct interrupts are even more rarely supplied.
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operations, to handle the input requirements. However, the user is prone to specifying clobbered registers incorrectly. The MSVC form of an embedded
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or other synchronization and locking primitives. Nearly every modern processor has these or similar instructions, as they are necessary to implement
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However it is possible to implement support for inline assembly without support from the compiler backend by using an external assembler instead.
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to the higher 8 bits of the register; and the jump is used to judge on whatever flag bit that happens to correspond to the FPU status bit.
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assembly. The former simply passes text verbatim to the assembler, while the latter performs some substitutions for register locations.
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for special instructions are provided by most compilers and C-function wrappers for arbitrary system calls are available on every
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81:
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compiler to understand instruction names, as the compiler is only needed to substitute its register assignments, plus a few
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To emit special directives for the linker or assembler, for example to change sectioning, macros, or to make symbol aliases.
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1282:
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The ISO C++ standard and ISO C standards (annex J) specify a conditionally supported syntax for inline assembler:
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Alternative facilities are often provided as a way to simplify the work for both the compiler and the programmer.
73:: Programmers can use assembly language code to implement the most performance-sensitive parts of their program's
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is used to make requests to the operating system. This is rarely a feature in a higher-level language, and so
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followed by conditional jump idiom is used to access the x87 FPU status word bits C0 and C2.
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The asm declaration is conditionally-supported; its meaning is implementation-defined.
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496:/* announce to the compiler that the memory and condition codes have been modified */
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to be embedded within a program, among code that otherwise has been compiled from a
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The FNSTSW AX form of the instruction is used primarily in conditional branching...
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There are, in general, two types of inline assembly supported by C/C++ compilers:
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The embedding of assembly language code is usually done for one of these reasons:
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17:
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74:
890:// No need to manually return anything as the value is already on FP stack
1215:
119:
39:
1042:"Migration and Compatibility Guide: Inline assembly with Arm Compiler 6"
959:""To a compiler, a blob of inline assembly is like a slap in the face.""
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122:
1154:
1211:
1086:
996:"Extended Asm - Assembler Instructions with C Expression Operands"
523:* wrappers return -1 on error and set the errno global variable */
108:
101:
295:
The following C code example shows an x86 system call wrapper in
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1205:
909:
stores the status in a general-purpose register; sahf sets the
1087:"⚙ D54891 [RFC] Checking inline assembly for validity"
621:
613:
520:/* The operating system will return a negative value on error;
1267:
100:. Examples of specialized instructions are found in the
1105:"LLVM Language Reference: Inline assembly expressions"
84:: Most processors offer special instructions, such as
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for system calls are written using inline assembler.
612:
shows code that computes the tangent of x using the
478:/* pass system call number in eax ("a") */
770:// C2 = 1: x is out of range, do argument reduction
259:GNAT (Ada language frontend of the GCC suite), and
901:For readers unfamiliar with x87 programming, the
8:
92:instructions which may be used to construct
27:Embedding assembly in a high-level language
1229:"FSTSW/FNSTSW — Store x87 FPU Status Word"
1053:
1051:
927:
925:
827:// C2 = 1: partial reminder, need to loop
415:/* return result in eax ("a") */
758:// if (!(fp_status & 0x20)) goto Lret
608:This example of inline assembly from the
990:
988:
517:/* these registers are clobbered too */
921:
704:// C0 = 1: x is NAN, infinity, or empty
460:/* pass arg3 in edx ("d") */
445:/* pass arg2 in ecx ("c") */
430:/* pass arg1 in ebx ("b") */
1058:d'Antras, Amanieu (13 December 2019).
1018:
1016:
42:that allows low-level code written in
7:
604:Processor-specific instruction in D
1060:"Rust RFC-2873: stable inline asm"
135:not yet supported by the compiler.
25:
1180:"LDC inline assembly expressions"
957:Striegel, Ben (13 January 2020).
707:// 387's can handle denormals
1075:Pull Request for status tracking
397:/* make the request to the OS */
242:used to have a similar facility.
178:An asm declaration has the form
1263:GCC Inline Assembler Reference
186:
1:
80:Access to processor-specific
740:// dump X, which is always 1
306:To recap, GCC supports both
629:// Compute the tangent of x
62:Motivation and alternatives
1299:
1134:Rust Documentation (1.0.0)
677:// test for oddball values
1248:GCC-Inline-Assembly-HOWTO
297:AT&T assembler syntax
263:uses the GCC syntax. The
1000:Using the GNU C Compiler
626:
316:
253:domain-specific language
848:// remove pi from stack
1159:D programming language
610:D programming language
394:"int $ 0x80"
265:D programming language
199:
1258:GNAT Inline Assembler
1253:Clang Inline assembly
800:// reminder (partial)
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169:In language standards
48:higher-level language
38:is a feature of some
280:A system call in GCC
32:computer programming
205:In actual compilers
154:Intrinsic functions
133:calling conventions
1283:Assembly languages
1155:"Inline Assembler"
1109:LLVM Documentation
1028:docs.microsoft.com
1024:"Inline Assembler"
933:"DontUseInlineAsm"
484:"memory"
131:Access to special
1268:Compiler Explorer
1130:"Inline Assembly"
290:wrapper functions
44:assembly language
16:(Redirected from
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511:"ebp"
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1218:– System Calls
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911:FLAGS register
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490:"cc"
448:"+d"
433:"+c"
418:"+b"
403:"=a"
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1214:Programmer's
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466:"a"
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301:GNU Assembler
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1199:
1187:. Retrieved
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1162:. Retrieved
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1149:
1137:. Retrieved
1133:
1124:
1112:. Retrieved
1108:
1099:
1090:
1081:
1070:
1063:. Retrieved
1036:
1027:
1003:. Retrieved
999:
978:
966:. Retrieved
962:
952:
940:. Retrieved
936:
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607:
311:
307:
305:
299:, using the
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245:
240:ARM Compiler
212:
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151:
147:
139:System calls
98:multitasking
90:Test and Set
82:instructions
71:Optimization
65:
35:
29:
1206:syscall(2)
1189:15 January
1164:15 January
1139:15 January
1114:15 January
1065:15 January
1005:15 January
968:15 January
942:21 January
917:References
903:fstsw-sahf
779:// load pi
541:&&
236:Visual C++
160:platform.
94:semaphores
75:algorithms
668:// load x
286:interrupt
40:compilers
1277:Category
937:GCC Wiki
334:syscall3
312:extended
275:Examples
194:) ;
120:Motorola
50:such as
863:trigerr
698:trigerr
388:__asm__
222:__asm__
123:Altivec
1216:Manual
1184:D Wiki
963:Reddit
869:return
794:fprem1
589:return
319:extern
164:Syntax
118:, and
1212:Linux
982:C++,
907:fstsw
803:fstsw
773:fldpi
743:fstsw
716:fptan
680:fstsw
559:errno
535:<=
325:errno
308:basic
232:__asm
224:) in
109:Intel
102:SPARC
34:, an
1191:2020
1166:2020
1141:2020
1116:2020
1067:2020
1007:2020
970:2020
944:2020
884:Lret
872:real
854:SC18
830:fstp
821:SC17
812:sahf
788:SC17
782:fxch
764:Lret
752:sahf
722:fstp
710:SC18
689:sahf
671:fxam
641:real
632:real
547:<
532:-125
454:arg3
439:arg2
424:arg1
370:arg3
361:arg2
352:arg1
310:and
261:LLVM
220:(or
158:Unix
114:and
88:and
878:nan
851:jmp
761:jnp
659:fld
653:asm
635:tan
622:x87
618:FPU
616:'s
614:x86
592:res
574:res
568:res
544:res
538:res
472:num
409:res
382:res
379:int
367:int
358:int
349:int
343:num
340:int
331:int
322:int
249:mov
226:GCC
218:asm
188:asm
116:SSE
112:MMX
105:VIS
56:Ada
54:or
30:In
1279::
1231:.
1182:.
1157:.
1132:.
1107:.
1089:.
1069:.
1050:^
1026:.
1015:^
998:.
987:^
961:.
935:.
924:^
833:ST
818:jp
806:AX
746:AX
725:ST
695:jc
683:AX
580:-1
526:if
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190:(
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896:}
893:;
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875:.
866::
860:}
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