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This is a follow-up to e81d015e.

In the RISC-V ABI, FP arguments to functions are passed in integer registers
(or pairs of integer registers) in two cases:
1- the FP argument is a variadic argument
2- the FP argument is a fixed argument but all 8 FP registers reserved for
   parameter passing have been used already.

The previous implementation handled only case 1, with some problems.

This commit implements both 1 and 2.  To this end, 8 extra FP
caller-save registers are used to hold the values of the FP arguments
that must be passed in integer registers.  Fixup code moves these FP
registers to integer registers / register pairs.  Symmetrically, at
function entry, the integer registers / register pairs are moved back
to the FP registers.

8 extra FP registers is enough because there are only 8 integer
registers used for parameter passing, so at most 8 FP arguments may
need to be moved to integer registers.

This reverts commit 4dfcd7d4.

The `__builtin_nop` function is documented only for PowerPC.
It was added to the other architectures by copy paste, but has no
known uses.  So, remove `__builtin_nop` from all architectures
but PowerPC.

When printing an extended asm code fragment, placeholders %n
are replaced by register names.

Currently we ignore the fact that some assemblers use different
register names depending on the width of the data that resides
in the register.

For example, x86_64 uses %rax for a 64-bit quantity and %eax for
a 32-bit quantity, but CompCert always prints %rax in extended asm
statements.  This is problematic if we want to use 32-bit integer
instructions in extended asm, e.g.

             int x, y;
             asm("addl %1, %0", "=r"(x), "r"(y));
produces
             addl %rax, %rdx

which is syntactically incorrect.

Another example is ARM FP registers: D0 is a double-precision float,
but S0 is a single-precision float.

This commit partially solves this issue by taking into account the
Cminor type of the asm parameter when printing the corresponding register.
Continuing the previous example,

             int x, y;
             asm("addl %1, %0", "=r"(x), "r"(y));
now produces
             addl %eax, %edx

This is not perfect yet: we use Cminor types, because this is all we
have at hand, and not source C types, hence "char" and "short" parameters
are still printed like "int" parameters, which is not good for x86.
(I.e. we produce %eax where GCC might have produced %al or %ax.)
We'll leave this issue open.

The option -fcommon controls whether uninitialized global
variables are placed in the COMMON section. If the option is given
in the negated form, -fno-common, variables are not placed in the
COMMON section. They are placed in the same sections as gcc does.

If the variables are not placed in the COMMON section merging of
tentative definitions is inhibited and multiple definitions lead
to a linker error, as it does for gcc.

* Generate a nop instruction after ais annotations.

In order to prevent the merging of ais annotations with following
Labels a nop instruction is inserted, but only if the annotation
is followed immediately by a label.

The insertion of nop instructions is performed during the
expansion of builtin and pseudo assembler instructions and is
processor independent, by inserting a __builtin_nop built-in.

* Add Pnop instruction to ARM, RISC-V, and x86

ARM as well as RISC-V don't have nop instructions that can
be easily encoded by for example add with zero instructions.
For x86 we used to use `mov X0, X0` for nop but this may
not be as efficient as the true nop instruction.

* Implement __builtin_nop on all supported target architectures.

This builtin is not yet made available on the C side for all architectures.

Bug 24067

x2 is the stack pointer of the riscV, both sp and x2 are supported
but to be safe use x2 in annotations.
Bug 23176

It should be 'esp' respectively 'rsp' for x86, 'r13' for arm and
'sp' for riscV.
Bug 23176.

The ais annotations are now handled in a separate file shared
between all architectures. Also two different variants of
replacements are supported, %e which expands to ais expressions
and %l which also expands to an ais expression but is guaranted to
be usable as l-value in the ais annotation. Otherwise the new
warning is Wrong_is_parameter is generated.

Also an error message is generated if floating point variables are
used in ais annotations since a3 does not support them at the
moment.

Additionally an error message is generated for plain volatile
variables used, since they will enforce a volatile load and result
in the value being passed to the annotation instead of the address
as other global variables.

Instead of using reset_constants use reset_literals which avoids
emptying the jumptables.
Bug 22525

Instead of just storing the constants in a list, they are now
stored in a hashtable. This avoids printing of duplicates.
Bug 22525

The new_label function is alway equal to PrintAsmaux.new_label.
Bug 22472

The ais annotations can be inserted via the new ais variants of
the builtin annotation. They mainly differe in that they have an
address format specifier '%addr' which will be replaced by the
adress in the binary.

The implementation simply prints a label for the builtin call
alongside a the text of the annotation as comment and inserts the
annotation together as acii string in a separate section
'ais_annotations' and replaces the usages of the address format
specifiers by the address of the label of the builtin call.

This commits adds code generation for the RISC-V architecture, both in 32- and 64-bit modes.

The generated code was lightly tested using the simulator and cross-binutils from https://riscv.org/software-tools/

This port required the following additional changes:

- Integers: More properties about shrx

- SelectOp: now provides smart constructors for mulhs and mulhu

- SelectDiv, 32-bit integer division and modulus: implement constant propagation, use the new smart constructors mulhs and mulhu.

- Runtime library: if no asm implementation is provided, run the reference C implementation through CompCert.  Since CompCert rejects the definitions of names of special functions such as __i64_shl, the reference implementation now uses "i64_" names, e.g. "i64_shl", and a renaming "i64_ -> __i64_" is performed over the generated assembly file, before assembling and building the runtime library.

- test/: add SIMU make variable to run tests through a simulator

- test/regression/alignas.c: make sure _Alignas and _Alignof are not #define'd by C headers

commit da14495c01cf4f66a928c2feff5c53f09bde837f
Author: Xavier Leroy <xavier.leroy@inria.fr>
Date:   Thu Apr 13 17:36:10 2017 +0200

    RISC-V port, continued

    Now working on Asmgen.

commit 36f36eb3a5abfbb8805960443d087b6a83e86005
Author: Xavier Leroy <xavier.leroy@inria.fr>
Date:   Wed Apr 12 17:26:39 2017 +0200

    RISC-V port, first steps

    This port is based on Prashanth Mundkur's experimental RV32 port and brings it up to date with CompCert, and adds 64-bit support (RV64).  Work in progress.