X-Git-Url: https://git.openssl.org/gitweb/?p=openssl.git;a=blobdiff_plain;f=crypto%2Fperlasm%2Fx86_64-xlate.pl;h=75092e058038dc62dd092445b2232a3e255d2197;hp=33ffc418a1e2dd19a237768273bfd9728f765fd3;hb=4f469342692d0b2804f466f82d6ad8fea604a647;hpb=1cfd258ed61721ef667ea8a6ca46b57f3765007e

diff --git a/crypto/perlasm/x86_64-xlate.pl b/crypto/perlasm/x86_64-xlate.pl
index 33ffc418a1..75092e0580 100755
--- a/crypto/perlasm/x86_64-xlate.pl
+++ b/crypto/perlasm/x86_64-xlate.pl
@@ -30,20 +30,55 @@
 #    stack frame allocation. If volatile storage is actually required
 #    that is. If not, just leave the stack as is.
 # 3. Functions tagged with ".type name,@function" get crafted with
-#    unified Windows prologue and epilogue automatically. If you want
+#    unified Win64 prologue and epilogue automatically. If you want
 #    to take care of ABI differences yourself, tag functions as
-#    ".type name,@abi-omnipotent."
-# 4. As minor optimization you can specify number of input arguments
-#    as ".type name,@function,N." Keep in mind that if N is larger
-#    than 6, then you *have to* write "abi-omnipotent" code, because
-#    it can't be resolved with unified prologue.
-# 5. Name local labels as .L*.
-# 6. Don't use repret, it's generated automatically.
+#    ".type name,@abi-omnipotent" instead.
+# 4. To optimize the Win64 prologue you can specify number of input
+#    arguments as ".type name,@function,N." Keep in mind that if N is
+#    larger than 6, then you *have to* write "abi-omnipotent" code,
+#    because >6 cases can't be addressed with unified prologue.
+# 5. Name local labels as .L*, do *not* use dynamic labels such as 1:
+#    (sorry about latter).
+# 6. Don't use [or hand-code with .byte] "rep ret." "ret" mnemonic is
+#    required to identify the spots, where to inject Win64 epilogue!
+#    But on the pros, it's then prefixed with rep automatically:-)
+# 7. Due to MASM limitations [and certain general counter-intuitivity
+#    of ip-relative addressing] generation of position-independent
+#    code is assisted by synthetic directive, .picmeup, which puts
+#    address of the *next* instruction into target register.
+#
+#    Example 1:
+#		.picmeup	%rax
+#		lea		.Label-.(%rax),%rax
+#    Example 2:
+#		.picmeup	%rcx
+#	.Lpic_point:
+#		...
+#		lea		.Label-.Lpic_point(%rcx),%rbp
 
 my $output = shift;
-open STDOUT,">$output" || die "can't open $output: $!";
 
-my $masm=1 if ($output =~ /\.asm/);
+{ my ($stddev,$stdino,@junk)=stat(STDOUT);
+  my ($outdev,$outino,@junk)=stat($output);
+
+    open STDOUT,">$output" || die "can't open $output: $!"
+	if ($stddev!=$outdev || $stdino!=$outino);
+}
+
+my $win64=1 if ($output =~ /\.asm/);
+
+my $masmref=8 + 50727*2**-32;	# 8.00.50727 shipped with VS2005
+my $masm=$masmref;
+my $PTR=" PTR";
+
+my $nasm=0;
+
+if ($win64)
+{   if ($ENV{ASM} =~ m/nasm/)
+    {	$nasm = 1; $PTR="";   }
+    elsif (`ml64 2>&1` =~ m/Version ([0-9]+)\.([0-9]+)(\.([0-9]+))?/)
+    {	$masm = $1 + $2*2**-16 + $4*2**-32;   }
+}
 
 my $current_segment;
 my $current_function;
@@ -54,16 +89,18 @@ my $current_function;
 	local	*line = shift;
 	undef	$ret;
 
-	if ($line =~ /^([a-z]+)/i) {
+	if ($line =~ /^([a-z][a-z0-9]*)/i) {
 	    $self->{op} = $1;
 	    $ret = $self;
 	    $line = substr($line,@+[0]); $line =~ s/^\s+//;
 
 	    undef $self->{sz};
-	    if ($self->{op} =~ /(movz)b.*/) {	# movz is pain...
+	    if ($self->{op} =~ /^(movz)b.*/) {	# movz is pain...
 		$self->{op} = $1;
 		$self->{sz} = "b";
-	    } elsif ($self->{op} =~ /([a-z]{3,})([qlwb])/) {
+	    } elsif ($self->{op} =~ /call/) {
+		$self->{sz} = ""
+	    } elsif ($self->{op} =~ /([a-z]{3,})([qlwb])$/) {
 		$self->{op} = $1;
 		$self->{sz} = $2;
 	    }
@@ -78,27 +115,32 @@ my $current_function;
     }
     sub out {
 	my $self = shift;
-	if (!$masm) {
-	    if ($self->{op} eq "movz") {	# movz in pain...
+	if (!$win64) {
+	    if ($self->{op} eq "movz") {	# movz is pain...
 		sprintf "%s%s%s",$self->{op},$self->{sz},shift;
+	    } elsif ($self->{op} =~ /^set/) { 
+		"$self->{op}";
 	    } elsif ($self->{op} eq "ret") {
 	    	".byte	0xf3,0xc3";
 	    } else {
 		"$self->{op}$self->{sz}";
 	    }
 	} else {
-	    $self->{op} =~ s/movz/movzx/;
+	    $self->{op} =~ s/^movz/movzx/;
 	    if ($self->{op} eq "ret") {
 		$self->{op} = "";
 		if ($current_function->{abi} eq "svr4") {
-		    $self->{op} = "mov	rdi,QWORD PTR 8[rsp]\t;WIN64 epilogue\n\t".
-				  "mov	rsi,QWORD PTR 16[rsp]\n\t";
+		    $self->{op} = "mov	rdi,QWORD${PTR}[8+rsp]\t;WIN64 epilogue\n\t".
+				  "mov	rsi,QWORD${PTR}[16+rsp]\n\t";
 	    	}
 		$self->{op} .= "DB\t0F3h,0C3h\t\t;repret";
+	    } elsif ($self->{op} =~ /^j/ && $nasm) {
+		$self->{op} .= " NEAR";
 	    }
 	    $self->{op};
 	}
     }
+    sub mnemonic { shift->{op}; }
 }
 { package const;	# pick up constants, which start with $
     sub re {
@@ -115,7 +157,17 @@ my $current_function;
     }
     sub out {
     	my $self = shift;
-	sprintf $masm?"%s":"\$%s",$self->{value};
+
+	if (!$win64) {
+	    # Solaris /usr/ccs/bin/as can't handle multiplications
+	    # in $self->{value}
+	    $self->{value} =~ s/(?<![0-9a-f])(0[x0-9a-f]+)/oct($1)/egi;
+	    $self->{value} =~ s/([0-9]+\s*[\*\/\%]\s*[0-9]+)/eval($1)/eg;
+	    sprintf "\$%s",$self->{value};
+	} else {
+	    $self->{value} =~ s/0x([0-9a-f]+)/0$1h/ig;
+	    sprintf "%s",$self->{value};
+	}
     }
 }
 { package ea;		# pick up effective addresses: expr(%reg,%reg,scale)
@@ -131,7 +183,6 @@ my $current_function;
 	    $ret = $self;
 	    $line = substr($line,@+[0]); $line =~ s/^\s+//;
 
-	    $self->{label} =~ s/\.L/\$L/g;
 	    $self->{base}  =~ s/^%//;
 	    $self->{index} =~ s/^%// if (defined($self->{index}));
 	}
@@ -142,26 +193,43 @@ my $current_function;
     	my $self = shift;
 	my $sz = shift;
 
-	if (!$masm) {
+	# Silently convert all EAs to 64-bit. This is required for
+	# elder GNU assembler and results in more compact code,
+	# *but* most importantly AES module depends on this feature!
+	$self->{index} =~ s/^[er](.?[0-9xpi])[d]?$/r\1/;
+	$self->{base}  =~ s/^[er](.?[0-9xpi])[d]?$/r\1/;
+
+	if (!$win64) {
+	    # Solaris /usr/ccs/bin/as can't handle multiplications
+	    # in $self->{label}
+	    $self->{label} =~ s/(?<![0-9a-f])(0[x0-9a-f]+)/oct($1)/egi;
+	    $self->{label} =~ s/([0-9]+\s*[\*\/\%]\s*[0-9]+)/eval($1)/eg;
+
 	    if (defined($self->{index})) {
-		sprintf "%s(%%%s,%%%s,%d)",	$self->{label},$self->{base},
+		sprintf "%s(%%%s,%%%s,%d)",
+					$self->{label},$self->{base},
 					$self->{index},$self->{scale};
-	    }
-	    else {
+	    } else {
 		sprintf "%s(%%%s)",	$self->{label},$self->{base};
 	    }
 	} else {
-	    %szmap = ( b=>"BYTE", w=>"WORD", l=>"DWORD", q=>"QWORD" );
+	    %szmap = ( b=>"BYTE$PTR", w=>"WORD$PTR", l=>"DWORD$PTR", q=>"QWORD$PTR" );
+
+	    $self->{label} =~ s/\./\$/g;
+	    $self->{label} =~ s/0x([0-9a-f]+)/0$1h/ig;
+	    $self->{label} = "($self->{label})" if ($self->{label} =~ /[\*\+\-\/]/);
 
 	    if (defined($self->{index})) {
-		sprintf "%s PTR %s[%s*%d+%s]",$szmap{$sz},
-					$self->{label},
+		sprintf "%s[%s%s*%d+%s]",$szmap{$sz},
+					$self->{label}?"$self->{label}+":"",
 					$self->{index},$self->{scale},
 					$self->{base};
-	    }
-	    else {
-		sprintf "%s PTR %s[%s]",$szmap{$sz},
-					$self->{label},$self->{base};
+	    } elsif ($self->{base} eq "rip") {
+		sprintf "%s[%s]",$szmap{$sz},$self->{label};
+	    } else {
+		sprintf "%s[%s%s]",$szmap{$sz},
+					$self->{label}?"$self->{label}+":"",
+					$self->{base};
 	    }
 	}
     }
@@ -198,7 +266,7 @@ my $current_function;
     }
     sub out {
     	my $self = shift;
-	sprintf $masm?"%s":"%%%s",$self->{value};
+	sprintf $win64?"%s":"%%%s",$self->{value};
     }
 }
 { package label;	# pick up labels, which end with :
@@ -212,32 +280,32 @@ my $current_function;
 	    $ret = $self;
 	    $line = substr($line,@+[0]); $line =~ s/^\s+//;
 
-	    $self->{value} =~ s/\.L/\$L/ if ($masm);
+	    $self->{value} =~ s/\.L/\$L/ if ($win64);
 	}
 	$ret;
     }
     sub out {
 	my $self = shift;
 
-	if (!$masm) {
+	if (!$win64) {
 	    $self->{value};
 	} elsif ($self->{value} ne "$current_function->{name}:") {
 	    $self->{value};
 	} elsif ($current_function->{abi} eq "svr4") {
-	    my $func =	"$current_function->{name}	PROC\n".
-			"	mov	QWORD PTR 8[rsp],rdi\t;WIN64 prologue\n".
-			"	mov	QWORD PTR 16[rsp],rsi\n";
+	    my $func =	"$current_function->{name}".($nasm?":":"\tPROC")."\n".
+			"	mov	QWORD${PTR}[8+rsp],rdi\t;WIN64 prologue\n".
+			"	mov	QWORD${PTR}[16+rsp],rsi\n";
 	    my $narg = $current_function->{narg};
 	    $narg=6 if (!defined($narg));
 	    $func .= "	mov	rdi,rcx\n" if ($narg>0);
 	    $func .= "	mov	rsi,rdx\n" if ($narg>1);
 	    $func .= "	mov	rdx,r8\n"  if ($narg>2);
 	    $func .= "	mov	rcx,r9\n"  if ($narg>3);
-	    $func .= "	mov	r8,QWORD PTR 40[rsp]\n" if ($narg>4);
-	    $func .= "	mov	r9,QWORD PTR 48[rsp]\n" if ($narg>5);
+	    $func .= "	mov	r8,QWORD${PTR}[40+rsp]\n" if ($narg>4);
+	    $func .= "	mov	r9,QWORD${PTR}[48+rsp]\n" if ($narg>5);
 	    $func .= "\n";
 	} else {
-	   "$current_function->{name}	PROC";
+	   "$current_function->{name}".($nasm?":":"\tPROC");
 	}
     }
 }
@@ -252,7 +320,7 @@ my $current_function;
 	    $ret = $self;
 	    $line = substr($line,@+[0]); $line =~ s/^\s+//;
 
-	    $self->{value} =~ s/\.L/\$L/g if ($masm);
+	    $self->{value} =~ s/\.L/\$L/g if ($win64);
 	}
 	$ret;
     }
@@ -267,13 +335,30 @@ my $current_function;
 	local	*line = shift;
 	undef	$ret;
 	my	$dir;
+	my	%opcode =	# lea 2f-1f(%rip),%dst; 1: nop; 2:
+		(	"%rax"=>0x01058d48,	"%rcx"=>0x010d8d48,
+			"%rdx"=>0x01158d48,	"%rbx"=>0x011d8d48,
+			"%rsp"=>0x01258d48,	"%rbp"=>0x012d8d48,
+			"%rsi"=>0x01358d48,	"%rdi"=>0x013d8d48,
+			"%r8" =>0x01058d4c,	"%r9" =>0x010d8d4c,
+			"%r10"=>0x01158d4c,	"%r11"=>0x011d8d4c,
+			"%r12"=>0x01258d4c,	"%r13"=>0x012d8d4c,
+			"%r14"=>0x01358d4c,	"%r15"=>0x013d8d4c	);
 
 	if ($line =~ /^\s*(\.\w+)/) {
-	    if (!$masm) {
+	    if (!$win64) {
 		$self->{value} = $1;
 		$line =~ s/\@abi\-omnipotent/\@function/;
 		$line =~ s/\@function.*/\@function/;
-		$self->{value} = $line;
+		if ($line =~ /\.picmeup\s+(%r[\w]+)/i) {
+		    $self->{value} = sprintf "\t.long\t0x%x,0x90000000",$opcode{$1};
+		} elsif ($line =~ /\.asciz\s+"(.*)"$/) {
+		    $self->{value} = ".byte\t".join(",",unpack("C*",$1),0);
+		} elsif ($line =~ /\.extern/) {
+		    $self->{value} = ""; # swallow extern
+		} else {
+		    $self->{value} = $line;
+		}
 		$line = "";
 		return $self;
 	    }
@@ -283,31 +368,42 @@ my $current_function;
 	    undef $self->{value};
 	    $line = substr($line,@+[0]); $line =~ s/^\s+//;
 	    SWITCH: for ($dir) {
-		/\.(text|data)/
+		/\.(text)/
 			    && do { my $v=undef;
-				    $v="$current_segment\tENDS\n" if ($current_segment);
-				    $current_segment = "_$1";
-				    $current_segment =~ tr/[a-z]/[A-Z]/;
-				    $v.="$current_segment\tSEGMENT PARA";
+				    if ($nasm) {
+					$v ="section	.$1 code align=64\n";
+					$v.="default	rel\n";
+					$v.="%define	PUBLIC global";
+				    } else {
+					$v="$current_segment\tENDS\n" if ($current_segment);
+					$current_segment = "_$1\$";
+					$current_segment =~ tr/[a-z]/[A-Z]/;
+					$v.="$current_segment\tSEGMENT ";
+					$v.=$masm>=$masmref ? "ALIGN(64)" : "PAGE";
+					$v.=" 'CODE'";
+				    }
 				    $self->{value} = $v;
 				    last;
 				  };
+		/\.extern/  && do { $self->{value}  = "EXTERN\t".$line;
+				    $self->{value} .= ":BYTE" if (!$nasm);
+				    last;
+				  };
 		/\.globl/   && do { $self->{value} = "PUBLIC\t".$line; last; };
 		/\.type/    && do { ($sym,$type,$narg) = split(',',$line);
-				    if ($type eq "\@function")
-				    {	undef $current_function;
+				    if ($type eq "\@function") {
+					undef $current_function;
 					$current_function->{name} = $sym;
 					$current_function->{abi}  = "svr4";
 					$current_function->{narg} = $narg;
-				    }
-				    elsif ($type eq "\@abi-omnipotent")
-				    {	undef $current_function;
+				    } elsif ($type eq "\@abi-omnipotent") {
+					undef $current_function;
 					$current_function->{name} = $sym;
 				    }
 				    last;
 				  };
-		/\.size/    && do { if (defined($current_function))
-				    {	$self->{value}="$current_function->{name}\tENDP";
+		/\.size/    && do { if (defined($current_function)) {
+					$self->{value}="$current_function->{name}\tENDP" if(!$nasm);
 					undef $current_function;
 				    }
 				    last;
@@ -317,11 +413,31 @@ my $current_function;
 			    && do { my @arr = split(',',$line);
 				    my $sz  = substr($1,0,1);
 				    my $last = pop(@arr);
+				    my $conv = sub  {	my $var=shift;
+							if ($var=~s/0x([0-9a-f]+)/0$1h/i) { $var; }
+							else { sprintf"0%Xh",$var; }
+						    };  
 
 				    $sz =~ tr/bvlq/BWDQ/;
 				    $self->{value} = "\tD$sz\t";
-				    for (@arr) { $self->{value} .= sprintf"0%Xh,",oct; }
-				    $self->{value} .= sprintf"0%Xh",oct($last);
+				    for (@arr) { $self->{value} .= &$conv($_).","; }
+				    $self->{value} .= &$conv($last);
+				    last;
+				  };
+		/\.picmeup/ && do { $self->{value} = sprintf"\tDD\t0%Xh,090000000h",$opcode{$line};
+				    last;
+				  };
+		/\.asciz/   && do { if ($line =~ /^"(.*)"$/) {
+					my @str=unpack("C*",$1);
+					push @str,0;
+					while ($#str>15) {
+					    $self->{value}.="DB\t"
+						.join(",",@str[0..15])."\n";
+					    foreach (0..15) { shift @str; }
+					}
+					$self->{value}.="DB\t"
+						.join(",",@str) if (@str);
+				    }
 				    last;
 				  };
 	    }
@@ -340,8 +456,9 @@ while($line=<>) {
 
     chomp($line);
 
-    $line =~ s/\[#!].*$//;	# get rid of comments...
-    $line =~ s/^\s+//;		# ... and skip white spaces
+    $line =~ s|[#!].*$||;	# get rid of asm-style comments...
+    $line =~ s|/\*.*\*/||;	# ... and C-style comments...
+    $line =~ s|^\s+||;		# ... and skip white spaces in beginning
 
     undef $label;
     undef $opcode;
@@ -373,16 +490,15 @@ while($line=<>) {
 	$sz=opcode->size();
 
 	if (defined($dst)) {
-	    if (!$masm) {
+	    if (!$win64) {
 		printf "\t%s\t%s,%s",	$opcode->out($dst->size()),
 					$src->out($sz),$dst->out($sz);
-	    }
-	    else {
+	    } else {
+		undef $sz if ($nasm && $opcode->mnemonic() eq "lea");
 		printf "\t%s\t%s,%s",	$opcode->out(),
 					$dst->out($sz),$src->out($sz);
 	    }
-	}
-	elsif (defined($src)) {
+	} elsif (defined($src)) {
 	    printf "\t%s\t%s",$opcode->out(),$src->out($sz);
 	} else {
 	    printf "\t%s",$opcode->out();
@@ -392,7 +508,7 @@ while($line=<>) {
     print $line,"\n";
 }
 
-print "\n$current_segment\tENDS\nEND\n" if ($masm);
+print "\n$current_segment\tENDS\nEND\n" if ($current_segment);
 
 close STDOUT;
 
@@ -433,7 +549,10 @@ close STDOUT;
 # arguments passed to callee, *but* not less than 4! This means that
 # upon function entry point 5th argument resides at 40(%rsp), as well
 # as that 32 bytes from 8(%rsp) can always be used as temporal
-# storage [without allocating a frame].
+# storage [without allocating a frame]. One can actually argue that
+# one can assume a "red zone" above stack pointer under Win64 as well.
+# Point is that at apparently no occasion Windows kernel would alter
+# the area above user stack pointer in true asynchronous manner...
 #
 # All the above means that if assembler programmer adheres to Unix
 # register and stack layout, but disregards the "red zone" existense,
@@ -457,3 +576,120 @@ close STDOUT;
 #	movq	16(%rsp),%rsi
 # endif
 #	ret
+#
+#################################################
+# Unlike on Unix systems(*) lack of Win64 stack unwinding information
+# has undesired side-effect at run-time: if an exception is raised in
+# assembler subroutine such as those in question (basically we're
+# referring to segmentation violations caused by malformed input
+# parameters), the application is briskly terminated without invoking
+# any exception handlers, most notably without generating memory dump
+# or any user notification whatsoever. This poses a problem. It's
+# possible to address it by registering custom language-specific
+# handler that would restore processor context to the state at
+# subroutine entry point and return "exception is not handled, keep
+# unwinding" code. Writing such handler can be a challenge... But it's
+# doable, though requires certain coding convention. Consider following
+# snippet:
+#
+# function:
+#	movq	%rsp,%rax	# copy rsp to volatile register
+#	pushq	%r15		# save non-volatile registers
+#	pushq	%rbx
+#	pushq	%rbp
+#	movq	%rsp,%r11
+#	subq	%rdi,%r11	# prepare [variable] stack frame
+#	andq	$-64,%r11
+#	movq	%rax,0(%r11)	# check for exceptions
+#	movq	%r11,%rsp	# allocate [variable] stack frame
+#	movq	%rax,0(%rsp)	# save original rsp value
+# magic_point:
+#	...
+#	movq	0(%rsp),%rcx	# pull original rsp value
+#	movq	-24(%rcx),%rbp	# restore non-volatile registers
+#	movq	-16(%rcx),%rbx
+#	movq	-8(%rcx),%r15
+#	movq	%rcx,%rsp	# restore original rsp
+#	ret
+#
+# The key is that up to magic_point copy of original rsp value remains
+# in chosen volatile register and no non-volatile register, except for
+# rsp, is modified. While past magic_point rsp remains constant till
+# the very end of the function. In this case custom language-specific
+# exception handler would look like this:
+#
+# EXCEPTION_DISPOSITION handler (EXCEPTION_RECORD *rec,ULONG64 frame,
+#		CONTEXT *context,DISPATCHER_CONTEXT *disp)
+# {	ULONG64 *rsp;
+#	if (context->Rip<magic_point)
+#	    rsp = (ULONG64 *)context->Rax;
+#	else
+#	{   rsp = ((ULONG64 **)context->Rsp)[0];
+#	    context->Rbp = rsp[-3];
+#	    context->Rbx = rsp[-2];
+#	    context->R15 = rsp[-1];
+#	}
+#	context->Rsp = (ULONG64)rsp;
+#	context->Rdi = rsp[1];
+#	context->Rsi = rsp[2];
+#
+#	memcpy (disp->ContextRecord,context,sizeof(CONTEXT));
+#	RtlVirtualUnwind(UNW_FLAG_NHANDLER,disp->ImageBase,
+#		dips->ControlPc,disp->FunctionEntry,disp->ContextRecord,
+#		&disp->HandlerData,&disp->EstablisherFrame,NULL);
+#	return ExceptionContinueSearch;
+# }
+#
+# It's appropriate to implement this handler in assembler, directly in
+# function's module. In order to do that one has to know members'
+# offsets in CONTEXT and DISPATCHER_CONTEXT structures and some constant
+# values. Here they are:
+#
+#	CONTEXT.Rax				120
+#	CONTEXT.Rcx				128
+#	CONTEXT.Rdx				136
+#	CONTEXT.Rbx				144
+#	CONTEXT.Rsp				152
+#	CONTEXT.Rbp				160
+#	CONTEXT.Rsi				168
+#	CONTEXT.Rdi				176
+#	CONTEXT.R8				184
+#	CONTEXT.R9				192
+#	CONTEXT.R10				200
+#	CONTEXT.R11				208
+#	CONTEXT.R12				216
+#	CONTEXT.R13				224
+#	CONTEXT.R14				232
+#	CONTEXT.R15				240
+#	CONTEXT.Rip				248
+#	sizeof(CONTEXT)				1232
+#	DISPATCHER_CONTEXT.ControlPc		0
+#	DISPATCHER_CONTEXT.ImageBase		8
+#	DISPATCHER_CONTEXT.FunctionEntry	16
+#	DISPATCHER_CONTEXT.EstablisherFrame	24
+#	DISPATCHER_CONTEXT.TargetIp		32
+#	DISPATCHER_CONTEXT.ContextRecord	40
+#	DISPATCHER_CONTEXT.LanguageHandler	48
+#	DISPATCHER_CONTEXT.HandlerData		56
+#	UNW_FLAG_NHANDLER			0
+#	ExceptionContinueSearch			1
+#
+# UNWIND_INFO structure for .xdata segment would be
+#	DB	9,0,0,0
+#	DD	imagerel handler
+# denoting exception handler for a function with zero-length prologue,
+# no stack frame or frame register.
+#
+# P.S.	Attentive reader can notice that effectively no exceptions are
+#	expected in "gear" prologue and epilogue [discussed in "ABI
+#	cross-reference" above]. No, there are not. This is because if
+#	memory area used by them was subject to segmentation violation,
+#	then exception would be raised upon call to our function and be
+#	accounted to caller and unwound from its frame, which is not a
+#	problem.
+#
+# (*)	Note that we're talking about run-time, not debug-time. Lack of
+#	unwind information makes debugging hard on both Windows and
+#	Unix. "Unlike" referes to the fact that on Unix signal handler
+#	will always be invoked, core dumped and appropriate exit code
+#	returned to parent (for user notification).