http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60206
Bug ID: 60206
Summary: IVOPT has no idea of inline asm
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: wmi at google dot com
CC: rguenth at gcc dot gnu.org, shenhan at google dot com
Host: i386
Target: i386
Created attachment 32141
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32141&action=edit
Testcase
This bug is found in google branch but I think the same problem also exists on
trunk (but not exposed).
For the testcase 1.c attached (1.c is extracted from libgcc/soft-fp/divtf3.c),
use trunk compiler gcc-r202164 (Target: x86_64-unknown-linux-gnu) + the patch
r204497 could expose the problem.
The command:
gcc -v -O2 -fno-omit-frame-pointer -fpic -c -S -m32 1.c
The error:
./1.c: In function ‘__divtf3’:
./1.c:64:1194: error: ‘asm’ operand has impossible constraints
The inline asm in error message is as follow:
do {
__asm__ (
"sub{l} {%11,%3|%3,%11}\n\t"
"sbb{l} {%9,%2|%2,%9}\n\t"
"sbb{l} {%7,%1|%1,%7}\n\t"
"sbb{l} {%5,%0|%0,%5}"
: "=r" ((USItype) (A_f[3])), "=&r" ((USItype) (A_f[2])), "=&r" ((USItype)
(A_f[1])), "=&r" ((USItype) (A_f[0])) : "0" ((USItype) (B_f[2])), "g"
((USItype) (A_f[2])), "1" ((USItype) (B_f[1])), "g" ((USItype) (A_f[1])), "2"
((USItype) (B_f[0])), "g" ((USItype) (A_f[0])), "3" ((USItype) (0)), "g"
((USItype) (_n_f[_i])));
} while ()
Because -fno-omit-frame-pointer is turned on and the command line uses -fpic,
there are only 5 registers for register allocation.
Before IVOPT,
%0, %1, %2, %3 require 4 registers. The index variable i of _n_f[_i] requires
another register. So 5 registers are used up here.
After IVOPT, MEM reference _n_f[_i] is converted to MEM[base: _874, index:
ivtmp.22_821, offset: 0B]. base and index require 2 registers, Now 6 registers
are required, so LRA cannot find enough registers to allocate.
trunk compiler doesn't expose the problem because of patch r202165. With patch
r202165, IVOPT doesn't change _n_f[_i] in inline asm above. But it just hided
the problem.
Should IVOPT care about the constraints in inline-asm and restrict its
optimization in some case?
