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Fix the FOR loop count formula (#1222)

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This commit is contained in:
Rangi
2023-11-07 17:43:46 -05:00
committed by GitHub
parent 5379e51094
commit 9fc088dcb0
4 changed files with 46 additions and 3 deletions
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10
src/asm/fstack.cpp
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@@ -240,7 +240,11 @@ bool yywrap(void)
// If this is a FOR, update the symbol value
if (contextStack->forName && fileInfo->iters[0] <= contextStack->nbReptIters) {
contextStack->forValue += contextStack->forStep;
// Avoid arithmetic overflow runtime error
uint32_t forValue = (uint32_t)contextStack->forValue +
(uint32_t)contextStack->forStep;
contextStack->forValue = forValue >= 0 ? (int32_t)forValue
: -(int32_t)~forValue - 1;
struct Symbol *sym = sym_AddVar(contextStack->forName,
contextStack->forValue);
@@ -513,9 +517,9 @@ void fstk_RunFor(char const *symName, int32_t start, int32_t stop, int32_t step,
uint32_t count = 0;
if (step > 0 && start < stop)
count = (stop - start - 1) / step + 1;
count = ((int64_t)stop - start - 1) / step + 1;
else if (step < 0 && stop < start)
count = (start - stop - 1) / -step + 1;
count = ((int64_t)start - stop - 1) / -(int64_t)step + 1;
else if (step == 0)
error("FOR cannot have a step value of 0\n");

19
test/asm/for-loop-count.asm Normal file
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@@ -0,0 +1,19 @@
; `FOR value, start, stop, step` calculates an initial count of loop repetitions as follows:
; - if (step > 0 && start < stop) then count = (stop - start - 1) / step + 1
; - if (step < 0 && stop < start) then count = (start - stop - 1) / -step + 1
; - else count = 0
; The absolute vaue |stop - start| needs to be 64-bit for this formula to work.
; Results may be compared with Python range(start, stop, step).
MACRO test
PRINTLN STRFMT("FOR x, %d, %d, %d", \1, \2, \3)
FOR x, \1, \2, \3
PRINTLN " {08x:x} = {d:x}"
ENDR
PRINTLN " done {08x:x} = {d:x}"
ENDM
test $8000_0000, $6000_0000, $4000_0000 ; [-2147483648, -1073741824, 0, 1073741824]
test $8000_0000, $7000_0000, $4400_0000 ; [-2147483648, -1006632960, 134217728, 1275068416]
test $7fff_ffff, $8000_0000, $8000_0000 ; [2147483647, -1]
test $8000_0000, $ffff_ffff, $8000_0000 ; []

2
test/asm/for-loop-count.err Normal file
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@@ -0,0 +1,2 @@
warning: for-loop-count.asm(19) -> for-loop-count.asm::test(12): [-Wbackwards-for]
FOR goes backwards from -2147483648 to -1 by -2147483648

18
test/asm/for-loop-count.out Normal file
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@@ -0,0 +1,18 @@
FOR x, -2147483648, 1610612736, 1073741824
80000000 = -2147483648
c0000000 = -1073741824
00000000 = 0
40000000 = 1073741824
done 80000000 = -2147483648
FOR x, -2147483648, 1879048192, 1140850688
80000000 = -2147483648
c4000000 = -1006632960
08000000 = 134217728
4c000000 = 1275068416
done 90000000 = -1879048192
FOR x, 2147483647, -2147483648, -2147483648
7fffffff = 2147483647
ffffffff = -1
done 7fffffff = 2147483647
FOR x, -2147483648, -1, -2147483648
done 80000000 = -2147483648