https://llvm.org/bugs/show_bug.cgi?id=26344
Bug ID: 26344
Summary: failure with unrolled loops which exit in the first
iteration.
Product: clang
Version: 3.7
Hardware: Macintosh
OS: MacOS X
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: lo...@phantasia.org
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
System: OSX 10.10.5
Xcode 7.2
gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr
--with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin14.5.0
Thread model: posix
Having a loop, which might exit after the first iteration, the optimization
fails to produce correct code.
<---
#define FAIL 0xFFFFFFFF
#define rotate_left(v, n) (v << n | v >> (32 - n))
unsigned int
encode_arm_immediate (unsigned int val)
{
unsigned int a, i;
for (i = 0; i < 32 ; i += 2)
if((a = rotate_left (val, i)) <= 0xFF)
return a | (i << 7) ; /* 12-bit pack: [shift-cnt,const]. */
return FAIL;
}
<---
calling this with val = 0xFF the loop should exit within the first iteration
with 0xFF
if this code fragment gets compiled with
clang -Wall -m32 -O2 test.c
or
clang -Wall -m64 -O2 test.c
the function will incorrectly return FAIL
looking at the assembler text generated on
clang -Wall -S -O2 test.c
shows the following:
<---
_encode_arm_immediate: ## @encode_arm_immediate
.cfi_startproc
## BB#0:
pushq %rbp
Ltmp4:
.cfi_def_cfa_offset 16
Ltmp5:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp6:
.cfi_def_cfa_register %rbp
movl %edi, %ecx
roll $2, %ecx
movl $256, %edx ## imm = 0x100
cmpl $255, %ecx
jbe LBB1_1
....
LBB1_1:
orl %edx, %ecx
movl %ecx, %eax
LBB1_2: ## %.loopexit5
popq %rbp
retq
.cfi_endproc
<---
it seems the first iteration with i=0 has been omited which causes the loop to
begin with i=1.
I attached the testcase source and the 2 (32bit, 64bit) generated files
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