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POThis maybe a bug of llvm-gcc for ios-armv7, that I can't explain
 

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  1. POThis maybe a bug of llvm-gcc for ios-armv7, that I can't explain
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    <pre> the following code works fine on linux-x86, darwin-x86, but not for ios-armv7. the right output should be: m[0]: 0.500000, v: 0.500000 m[1]: 0.500000, v: 0.500000 m[2]: 0.500000, v: 0.500000 m[3]: 0.500000, v: 0.500000 m[4]: 0.500000, v: 0.500000 but I found the wrong output: m[0]: 0.500000, v: 0.500000 m[1]: 0.500000, v: 0.000000 m[2]: 0.500000, v: 0.000000 m[3]: 0.500000, v: 0.000000 m[4]: 0.500000, v: 0.000000 I also found the stange when it's built for ios-armv7: [a] remove function 'func', move the function body to 'main' function, it works fine [b] declare the array 'm[5]' as 'double m[5]', it works fine [c] set the variable 'v' as 'v = 0.5 or v = sqrt(2.0f/8)', it works fine [d] if the gcc optimize option is '-O0', it works fine, but when it's '-O1 or -O2', wrong output occurs My iPad1 was cracked, so I can cross-compile a executable on my MacBook Air, and 'scp' the executable to iPad1 and run it. The following is details: 1. cross-compile a executable on Mac: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/llvm-gcc -O1 -Wall -arch armv7 -mcpu=cortex-a8 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.1.sdk -I/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.1.sdk/usr/include -D__IPHONE_OS__ -miphoneos-version-min=4.0 foo.c 2. 'scp' the executable to iPad scp a.out mobile@192.168.1.106:~ 3. 'ssh' to iPad ssh mobile@192.168.1.106 #the default password is 'alpine' 4. run a.out on iPad ./a.out </pre> <pre><code>#include &lt;stdio.h&gt; #include &lt;math.h&gt; int func(int n) /* [a] */ { int i; float m[5]; /* [b] */ double v; v = sqrt(2.0f/n); /* [c] */ for(i=0;i&lt;5;++i) { m[i]=v; printf("m[%d]: %f, v: %f\n", i, m[i], v); } return 0; } int main(int argc, char **argv) { return func(8); } </code></pre> <p>You can also find the whole code on <a href="https://gist.github.com/ashun/5992120" rel="nofollow">https://gist.github.com/ashun/5992120</a></p> <p>The following is the assembly. you can find the difference with the help of command 'vim -d'</p> <ol> <li>assembly of the previous code, declare the array 'm[5]' as 'double m[5]'</li> </ol> <pre> .section __TEXT,__text,regular,pure_instructions .section __TEXT,__textcoal_nt,coalesced,pure_instructions .section __TEXT,__const_coal,coalesced .section __TEXT,__picsymbolstub4,symbol_stubs,none,16 .section __TEXT,__StaticInit,regular,pure_instructions .syntax unified .section __TEXT,__text,regular,pure_instructions .globl _func .align 2 .code 16 .thumb_func _func _func: push {r4, r5, r6, r7, lr} add r7, sp, #12 str r8, [sp, #-4]! sub sp, #8 vmov.f32 s0, #2.000000e+00 movw r8, :lower16:(L_.str-(LPC0_0+4)) vmov s2, r0 movt r8, :upper16:(L_.str-(LPC0_0+4)) vcvt.f32.s32 d1, d1 LPC0_0: add r8, pc movs r4, #0 vdiv.f32 s0, s0, s2 vsqrt.f32 s0, s0 vcvt.f64.f32 d16, s0 vmov r5, r6, d16 LBB0_1: mov r1, r4 mov r0, r8 mov r2, r5 mov r3, r6 vstr.64 d16, [sp] adds r4, #1 blx _printf cmp r4, #5 bne LBB0_1 movs r0, #0 add sp, #8 ldr r8, [sp], #4 pop {r4, r5, r6, r7, pc} .globl _main .align 2 .code 16 .thumb_func _main _main: push {r7, lr} mov r7, sp movs r0, #8 bl _func movs r0, #0 pop {r7, pc} .section __TEXT,__cstring,cstring_literals L_.str: .asciz "m[%d]: %f, v: %f\n" .subsections_via_symbols </pre> <ol> <li>assembly of the previous code, declare the array 'm[5]' as 'double m[5]'</li> </ol> <pre> .section __TEXT,__text,regular,pure_instructions .section __TEXT,__textcoal_nt,coalesced,pure_instructions .section __TEXT,__const_coal,coalesced .section __TEXT,__picsymbolstub4,symbol_stubs,none,16 .section __TEXT,__StaticInit,regular,pure_instructions .syntax unified .section __TEXT,__text,regular,pure_instructions .globl _func .align 2 .code 16 .thumb_func _func _func: push {r4, r5, r6, r7, lr} add r7, sp, #12 str r8, [sp, #-4]! **vpush {d8}** sub sp, #8 vmov.f32 s0, #2.000000e+00 movw r8, :lower16:(L_.str-(LPC0_0+4)) vmov s2, r0 movt r8, :upper16:(L_.str-(LPC0_0+4)) vcvt.f32.s32 d1, d1 LPC0_0: add r8, pc movs r4, #0 vdiv.f32 s0, s0, s2 vcvt.f64.f32 d16, s0 vsqrt.f64 d8, d16 vmov r5, r6, d8 LBB0_1: mov r1, r4 mov r0, r8 mov r2, r5 mov r3, r6 vstr.64 d8, [sp] adds r4, #1 blx _printf cmp r4, #5 bne LBB0_1 movs r0, #0 add sp, #8 vpop {d8} ldr r8, [sp], #4 pop {r4, r5, r6, r7, pc} .globl _main .align 2 .code 16 .thumb_func _main _main: push {r7, lr} mov r7, sp movs r0, #8 bl _func movs r0, #0 pop {r7, pc} .section __TEXT,__cstring,cstring_literals L_.str: .asciz "m[%d]: %f, v: %f\n" .subsections_via_symbols </pre>
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    1. USaaashun
    1. USaaashun
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    1. COThat does look like a bug in the compiler indeed. Maybe you can add the generated assembly of `func` for the `-O1` case. The loop may be unrolled in which case replacing `5` by `2` may be more appropriate, if that still reproduces the bug.
      singulars
      1. POThis maybe a bug of llvm-gcc for ios-armv7, that I can't explain
      1. USBryan Olivier
    2. COThanks, I add the assembly of the sample code.
      singulars
      1. POThis maybe a bug of llvm-gcc for ios-armv7, that I can't explain
      1. USaaashun
 

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