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Fix fallback paths in fast_long_{add,sub}_function #17666

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Fix fallback paths in fast_long_{add,sub}_function #17666

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54324c9
Fix fallback paths in fast_long_{add,sub}_function
nielsdos Feb 1, 2025
54a5d2f
Not needed to change this
nielsdos Feb 2, 2025
2b762af
Perform int math once
nielsdos Feb 3, 2025
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14 changes: 10 additions & 4 deletions Zend/zend_operators.h
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Original file line number Diff line number Diff line change
Expand Up @@ -49,7 +49,7 @@
#include "zend_multiply.h"
#include "zend_object_handlers.h"

#define LONG_SIGN_MASK ZEND_LONG_MIN
#define LONG_SIGN_MASK (((zend_ulong)1) << (8*sizeof(zend_long)-1))
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The spec is complicated, but I'm not sure this is actually undefined behavior. FWIU, & is well defined for both signed and unsigned integers. In the case of two integers with the same conversion rank, the signed integer will be converted to an unsigned one.

In any case, I find the new and explicit solution a bit more obvious.

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The old code (((zend_long)1) << (8*sizeof(zend_long)-1)) (note zend_ulong->zend_long next to the 1) was UB. UBSAN complained at this computation.

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@iluuu1994 iluuu1994 Feb 2, 2025 •
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The old yes, but the new doesn't. ZEND_LONG_MIN should be equivalent to only the signed bit being set. The only difference between the old and new solution is the signedness, if I understand correctly. Anyway, this looks fine, this wasn't meant as an argument, just trying to understand. 🙂

Edit: I guess ZEND_LONG_MIN is only correct for 2's complement, which is the only binary representation in practice. I'm not sure if this is the only case where we rely on 2's complement, but as mentioned, the new solution is fine too.

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@nielsdos nielsdos Feb 2, 2025 •
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Ah right, I guess I mixed up ZEND_LONG_MIN with 32/64 bit longs... I'll change that back (to keep the diff as small as possible).

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@iluuu1994 iluuu1994 Feb 2, 2025 •
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In theory, your new solution is better because it supports all integer representations supported by C. In practice, it likely doesn't matter.

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Maybe so. However, we rely on 2s complement in a lot of other places, can you even get a 1s complement system somewhere nowadays?

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Absolutely no clue. 🙂

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We could add a respective check to configure.ac.


BEGIN_EXTERN_C()
ZEND_API zend_result ZEND_FASTCALL add_function(zval *result, zval *op1, zval *op2);
Expand Down Expand Up @@ -723,7 +723,7 @@ overflow: ZEND_ATTRIBUTE_COLD_LABEL
*/

if (UNEXPECTED((Z_LVAL_P(op1) & LONG_SIGN_MASK) == (Z_LVAL_P(op2) & LONG_SIGN_MASK)
&& (Z_LVAL_P(op1) & LONG_SIGN_MASK) != ((Z_LVAL_P(op1) + Z_LVAL_P(op2)) & LONG_SIGN_MASK))) {
&& (Z_LVAL_P(op1) & LONG_SIGN_MASK) != (((zend_ulong) Z_LVAL_P(op1) + (zend_ulong) Z_LVAL_P(op2)) & LONG_SIGN_MASK))) {
ZVAL_DOUBLE(result, (double) Z_LVAL_P(op1) + (double) Z_LVAL_P(op2));
} else {
ZVAL_LONG(result, Z_LVAL_P(op1) + Z_LVAL_P(op2));
Expand Down Expand Up @@ -804,11 +804,17 @@ overflow: ZEND_ATTRIBUTE_COLD_LABEL
ZVAL_LONG(result, llresult);
}
#else
ZVAL_LONG(result, Z_LVAL_P(op1) - Z_LVAL_P(op2));
/*
* 'result' may alias with op1 or op2, so we need to
* ensure that 'result' is not updated until after we
* have read the values of op1 and op2.
*/

if (UNEXPECTED((Z_LVAL_P(op1) & LONG_SIGN_MASK) != (Z_LVAL_P(op2) & LONG_SIGN_MASK)
&& (Z_LVAL_P(op1) & LONG_SIGN_MASK) != (Z_LVAL_P(result) & LONG_SIGN_MASK))) {
&& (Z_LVAL_P(op1) & LONG_SIGN_MASK) != (((zend_ulong) Z_LVAL_P(op1) - (zend_ulong) Z_LVAL_P(op2)) & LONG_SIGN_MASK))) {
ZVAL_DOUBLE(result, (double) Z_LVAL_P(op1) - (double) Z_LVAL_P(op2));
} else {
ZVAL_LONG(result, Z_LVAL_P(op1) - Z_LVAL_P(op2));
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I guess we're not actually testing this branch? Probably would only be used on some pretty esoteric platforms.

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I guess we're not actually testing this branch?

My bad (#17472) ;)

Probably would only be used on some pretty esoteric platforms.

Possibly not even that. Would require a compiler without ASM goto support, no builtins (SADDL etc.) and non Windows system. Plus, that compiler would need to support C11 (or at least the subset we're using).

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It's better to perform the integer math once, keeping the result in a temporary variable.
Otherwise C compiler may generate longer fast path code.

}
#endif
}
Expand Down
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