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Fix problem where array index computations involving a negative variable part of the index could be miscomputed when using the small memory model.

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This introduces a function to check whether the index portion of a pc_ixa intermediate code operation (used for array indexing) may be negative. This is also used when generating code for the large memory model, which can allow slightly more efficient code to be generated in some cases.

This fixes #45.
This commit is contained in:
Stephen Heumann 2017-11-02 00:45:47 -05:00
parent 763c5192df
commit 3aed2eb8ac
1 changed files with 34 additions and 2 deletions
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36
Gen.pas
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@ -2373,6 +2373,7 @@ case optype of
end; {GenInd}
procedure GenIxa (op: icptr);
{ Generate code for a pc_ixa }
@ -2408,6 +2409,29 @@ var
end; {Index}
function IndexCanBeNegative: boolean;
{ Check if the index value (right argument) of a pc_ixa }
{ can validly be negative. Returns false if this is }
{ precluded by the type of the operation or other available }
{ information, or if any use of a negative index would be }
{ undefined behavior. Otherwise, returns true. }
{ }
{ parameters: }
{ op - pc_ixa operation }
begin {IndexCanBeNegative}
IndexCanBeNegative := true;
if op^.optype in [cgUByte,cgUWord] then
IndexCanBeNegative := false
else if (op^.right^.opcode = pc_ldc) and (op^.right^.q >= 0) then
IndexCanBeNegative := false
else if (op^.left^.opcode in [pc_lao,pc_lda]) and (op^.left^.q = 0) then
{Can't index before start of array, so using a negative index would be UB}
IndexCanBeNegative := false;
end; {IndexCanBeNegative}
begin {GenIxa}
if smallMemoryModel then begin
lLong := gLong;
@ -2507,6 +2531,14 @@ if smallMemoryModel then begin
LoadX(op^.right);
gLong.fixedDisp := false;
end; {else}
if gLong.where = globalLabel then
if IndexCanBeNegative then begin
GenImplied(m_txa);
GenImplied(m_clc);
GenNative(m_adc_imm, immediate, gLong.disp, nil, 0);
GenImplied(m_tax);
gLong.disp := 0;
end; {if}
if (lLong.preference & gLong.where) = 0 then begin
if (lLong.preference & inPointer) <> 0 then begin
if gLong.where = localAddress then begin
@ -2598,12 +2630,12 @@ if smallMemoryModel then begin
otherwise:
Error(cge1);
end; {case}
end {if smallMemoryModel or (op^.right^.opcode = pc_ldc)}
end {if smallMemoryModel}
else begin
gLong.preference := onStack;
GenTree(op^.left);
GenTree(op^.right);
if op^.optype in [cgByte,cgWord] then begin
if IndexCanBeNegative then begin
lab1 := GenLabel;
GenNative(m_ldx_imm, immediate, $0000, nil, 0);
GenImplied(m_tay);