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Add a new optimize flag for FP math optimizations that break IEEE rules.

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There were several existing optimizations that could change behavior in ways that violated the IEEE standard with regard to infinities, NaNs, or signed zeros. They are now gated behind a new #pragma optimize flag. This change allows intermediate code peephole optimization and common subexpression elimination to be used while maintaining IEEE conformance, but also keeps the rule-breaking optimizations available if desired.

See section F.9.2 of recent C standards for a discussion of how these optimizations violate IEEE rules.
This commit is contained in:
Stephen Heumann 2021-11-29 20:31:15 -06:00
parent 6fa294aa3b
commit b43036409e
7 changed files with 61 additions and 14 deletions
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21
CGC.asm
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@ -252,3 +252,24 @@ maxLabel equ 3200
stz intLabel intLabel := 0;
rtl
end
datachk on
****************************************************************
*
* function SignBit (val: extended): integer;
*
* returns the sign bit of a floating-point number
* (0 for positive, 1 for negative)
*
****************************************************************
*
SignBit start cg
subroutine (10:val),0
asl val+8
stz val
rol val
return 2:val
end

6
CGC.pas
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@ -107,6 +107,12 @@ procedure InitLabels; extern;
{ }
{ Note: also defined in CGI.pas }
function SignBit (val: extended): integer; extern;
{ returns the sign bit of a floating-point number }
{ (0 for positive, 1 for negative) }
{-- These routines are defined in the compiler, but used from cg --}
function Calloc (bytes: integer): ptr; extern;

2
CGI.pas
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@ -327,6 +327,7 @@ var
debugFlag: boolean; {generate debugger calls?}
debugStrFlag: boolean; {gsbug/niftylist debug names?}
dataBank: boolean; {save, restore data bank?}
fastMath: boolean; {do FP math opts that break IEEE rules?}
floatCard: integer; {0 -> SANE; 1 -> FPE}
floatSlot: integer; {FPE slot}
loopOptimizations: boolean; {do loop optimizations?}
@ -817,6 +818,7 @@ volatile := false; {no volatile qualifiers found}
registers := cLineOptimize; {don't do register optimizations}
peepHole := cLineOptimize; {not doing peephole optimization (yet)}
npeepHole := cLineOptimize;
fastMath := cLineOptimize;
commonSubexpression := cLineOptimize; {not doing common subexpression elimination}
loopOptimizations := cLineOptimize; {not doing loop optimizations, yet}

30
DAG.pas
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@ -198,7 +198,9 @@ else if (op1 <> nil) and (op2 <> nil) then
CodesMatch := true;
cgReal, cgDouble, cgComp, cgExtended:
if op1^.rval = op2^.rval then
CodesMatch := true;
if (SignBit(op1^.rval) = SignBit(op2^.rval))
or fastMath then
CodesMatch := true;
cgString:
CodesMatch := LongStrCmp(op1^.str, op2^.str);
cgVoid, ccPointer:
@ -899,8 +901,9 @@ case op^.opcode of {check for optimizations of this node}
if op^.left^.opcode = pc_ldc then
ReverseChildren(op);
if op^.right^.opcode = pc_ldc then begin
if op^.right^.rval = 0.0 then
opv := op^.left;
if fastMath then
if op^.right^.rval = 0.0 then
opv := op^.left;
end; {if}
end; {else}
end; {case pc_adr}
@ -2069,8 +2072,9 @@ case op^.opcode of {check for optimizations of this node}
if rval = 1.0 then
opv := op^.left
else if rval = 0.0 then
if not SideEffects(op^.left) then
opv := op^.right;
if fastMath then
if not SideEffects(op^.left) then
opv := op^.right;
end; {if}
end; {else}
end; {case pc_mpr}
@ -2346,15 +2350,17 @@ case op^.opcode of {check for optimizations of this node}
op^.left^.rval := op^.left^.rval - op^.right^.rval;
opv := op^.left;
end {if}
else if op^.left^.rval = 0.0 then begin
op^.opcode := pc_ngr;
op^.left := op^.right;
op^.right := nil;
end; {else if}
else if op^.left^.rval = 0.0 then
if fastMath then begin
op^.opcode := pc_ngr;
op^.left := op^.right;
op^.right := nil;
end; {if}
end {if}
else if op^.right^.opcode = pc_ldc then begin
if op^.right^.rval = 0.0 then
opv := op^.left;
if fastMath then
if op^.right^.rval = 0.0 then
opv := op^.left;
end; {if}
end; {case pc_sbr}

6
Header.pas
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@ -18,7 +18,7 @@ uses CCommon, MM, Scanner, Symbol, CGI;
{$segment 'SCANNER'}
const
symFileVersion = 18; {version number of .sym file format}
symFileVersion = 19; {version number of .sym file format}
var
inhibitHeader: boolean; {should .sym includes be blocked?}
@ -845,7 +845,8 @@ procedure EndInclude {chPtr: ptr};
| (ord(saveStack) << 3)
| (ord(commonSubexpression) << 4)
| (ord(loopOptimizations) << 5)
| (ord(strictVararg) << 6));
| (ord(strictVararg) << 6)
| (ord(fastMath) << 7));
p_stacksize: WriteWord(stackSize);
@ -1508,6 +1509,7 @@ var
commonSubexpression := odd(val >> 4);
loopOptimizations := odd(val >> 5);
strictVararg := odd(val >> 6);
fastMath := odd(val >> 7);
end;
p_stacksize: stackSize := ReadWord;

2
Scanner.pas
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@ -3139,6 +3139,7 @@ if ch in ['a','d','e','i','l','p','u','w'] then begin
{ 16 - common subexpression elimination }
{ 32 - loop invariant removal }
{ 64 - remove stack checks for vararg calls}
{ 128 - fp math opts that break IEEE rules }
FlagPragmas(p_optimize);
NumericDirective;
if expressionType^.kind = scalarType then
@ -3152,6 +3153,7 @@ if ch in ['a','d','e','i','l','p','u','w'] then begin
commonSubexpression := odd(val >> 4);
loopOptimizations := odd(val >> 5);
strictVararg := not odd(val >> 6);
fastMath := odd(val >> 7);
if saveStack then
npeepHole := false;
if token.kind <> eolsy then

8
cc.notes
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@ -492,6 +492,14 @@ Using the extended format provides greater precision and range than the float or
If you want to get a value strictly in a certain type with no extra range or precision, you can store it in a variable of that type or explicitly cast it to that type. (In older versions of ORCA/C, floating-point casts did not remove extra range or precision, but now they do, as required by the C standards.)
Floating-Point Optimizations
----------------------------
ORCA/C can perform certain optimizations on floating-point computations based on properties that are true for real numbers but are not always true in the IEEE floating-point arithmetic system. In particular, these optimizations can occasionally cause behavior that differs from the IEEE standard in regard to infinities, NaNs, or the sign of zero. Historically, these optimizations were performed mainly as part of intermediate code peephole optimization and in some cases also as part of common subexpression elimination.
A new #pragma optimize bit has now been introduced to control this behavior. Setting bit 7 (a value of 128) allows floating-point math optimizations that may violate the IEEE standard. It currently only has an effect if #pragma optimize bit 0 or bit 4 is also set. If bit 7 is not set, these floating-point optimizations will not be performed. This allows most aspects of intermediate code peephole optimization and common subexpression elimination to be used while preserving IEEE floating-point behavior.
Additions to #pragma ignore
---------------------------
Several additional #pragma ignore bits are now supported.