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POIs GCC broken when taking the address of an argument on ARM7TDMI?
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copied!<p>My C code snippet takes the address of an argument and stores it in a volatile memory location (preprocessed code):</p> <pre><code>void foo(unsigned int x) { *(volatile unsigned int*)(0x4000000 + 0xd4) = (unsigned int)(&x); } int main() { foo(1); while(1); } </code></pre> <p>I used an SVN version of GCC for compiling this code. At the end of function <code>foo</code> I would expect to have the value <code>1</code> stored in the stack and, at <code>0x40000d4</code>, an address pointing to that value. When I compile without optimizations using the flag <code>-O0</code>, I get the expected ARM7TMDI assembly output (commented for your convenience):</p> <pre><code> .align 2 .global foo .type foo, %function foo: @ Function supports interworking. @ args = 0, pretend = 0, frame = 8 @ frame_needed = 0, uses_anonymous_args = 0 @ link register save eliminated. sub sp, sp, #8 str r0, [sp, #4] @ 3. Store the argument on the stack mov r3, #67108864 add r3, r3, #212 add r2, sp, #4 @ 4. Address of the stack variable str r2, [r3, #0] @ 5. Store the address at 0x40000d4 add sp, sp, #8 bx lr .size foo, .-foo .align 2 .global main .type main, %function main: @ Function supports interworking. @ args = 0, pretend = 0, frame = 0 @ frame_needed = 0, uses_anonymous_args = 0 stmfd sp!, {r4, lr} mov r0, #1 @ 1. Pass the argument in register 0 bl foo @ 2. Call function foo .L4: b .L4 .size main, .-main .ident "GCC: (GNU) 4.4.0 20080820 (experimental)" </code></pre> <p>It clearly stores the argument first on the stack and from there stores it at <code>0x40000d4</code>. When I compile with optimizations using <code>-O1</code>, I get something unexpected:</p> <pre><code> .align 2 .global foo .type foo, %function foo: @ Function supports interworking. @ args = 0, pretend = 0, frame = 8 @ frame_needed = 0, uses_anonymous_args = 0 @ link register save eliminated. sub sp, sp, #8 mov r2, #67108864 add r3, sp, #4 @ 3. Address of *something* on the stack str r3, [r2, #212] @ 4. Store the address at 0x40000d4 add sp, sp, #8 bx lr .size foo, .-foo .align 2 .global main .type main, %function main: @ Function supports interworking. @ args = 0, pretend = 0, frame = 0 @ frame_needed = 0, uses_anonymous_args = 0 stmfd sp!, {r4, lr} mov r0, #1 @ 1. Pass the argument in register 0 bl foo @ 2. Call function foo .L4: b .L4 .size main, .-main .ident "GCC: (GNU) 4.4.0 20080820 (experimental)" </code></pre> <p>This time the argument is never stored on the stack even though <em>something</em> from the stack is still stored at <code>0x40000d4</code>.</p> <p>Is this just expected/undefined behaviour? Have I done something wrong or have I in fact found a Compiler Bug™?</p>

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