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real: fix encoding of negative IEEE double/quad values [PR98216]

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In encode_ieee_double/quad, the assignment

  unsigned long WORD = r->sign << 31;

is intended to set the 31st bit of WORD whenever the sign bit is set.
But on LP64 hosts it also unintentionally sets the upper 32 bits of WORD,
because r->sign gets promoted from unsigned:1 to int and then the result
of the shift (equal to INT_MIN) gets sign extended from int to long.

In the C++ frontend, this bug causes incorrect mangling of negative
floating point values because the output of real_to_target called from
write_real_cst unexpectedly has the upper 32 bits of this word set,
which the caller doesn't mask out.

This patch fixes this by avoiding the unwanted sign extension.  Note
that r0-53976 fixed the same bug in encode_ieee_single long ago.

	PR c++/98216
	PR c++/91292

gcc/ChangeLog:

	* real.c (encode_ieee_double): Avoid unwanted sign extension.
	(encode_ieee_quad): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-float2.C: New test.
This commit is contained in:
Patrick Palka 2021-09-24 12:36:26 -04:00
parent 51ca050319
commit 34947d4e97
2 changed files with 18 additions and 2 deletions
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6
gcc/real.c
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@ -3150,9 +3150,10 @@ encode_ieee_double (const struct real_format *fmt, long *buf,
const REAL_VALUE_TYPE *r)
{
unsigned long image_lo, image_hi, sig_lo, sig_hi, exp;
unsigned long sign = r->sign;
bool denormal = (r->sig[SIGSZ-1] & SIG_MSB) == 0;
image_hi = r->sign << 31;
image_hi = sign << 31;
image_lo = 0;
if (HOST_BITS_PER_LONG == 64)
@ -3938,10 +3939,11 @@ encode_ieee_quad (const struct real_format *fmt, long *buf,
const REAL_VALUE_TYPE *r)
{
unsigned long image3, image2, image1, image0, exp;
unsigned long sign = r->sign;
bool denormal = (r->sig[SIGSZ-1] & SIG_MSB) == 0;
REAL_VALUE_TYPE u;
image3 = r->sign << 31;
image3 = sign << 31;
image2 = 0;
image1 = 0;
image0 = 0;

14
gcc/testsuite/g++.dg/cpp2a/nontype-float2.C Normal file
View File

@ -0,0 +1,14 @@
// PR c++/98216
// PR c++/91292
// { dg-do compile { target c++20 } }
template<auto> void f() { }
template void f<-1.0f>();
template void f<-2.0f>();
template void f<-1.0>();
template void f<-2.0>();
template void f<-1.0L>();
template void f<-2.0L>();