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copied!<p>It is <em>certain</em> that at least one C++ compiler will recognize the situation (when the 0 is known at compile time) and make it a no-op:</p> <p><strong>Source</strong></p> <pre><code>inline int shift( int what, int bitcount) { return what >> bitcount ; } int f() { return shift(42,0); } </code></pre> <p><strong>Compiler switches</strong></p> <pre><code>icpc -S -O3 -mssse3 -fp-model fast=2 bitsh.C </code></pre> <p><strong>Intel C++ 11.0 assembly</strong></p> <pre><code># -- Begin _Z1fv # mark_begin; .align 16,0x90 .globl _Z1fv _Z1fv: ..B1.1: # Preds ..B1.0 movl $42, %eax #7.10 ret #7.10 .align 16,0x90 # LOE # mark_end; .type _Z1fv,@function .size _Z1fv,.-_Z1fv .data # -- End _Z1fv .data .section .note.GNU-stack, "" # End </code></pre> <p>As you can see at ..B1.1, Intel compiles "return shift(42,0)" to "return 42."</p> <p>Intel 11 also culls the shift for these two variations:</p> <pre><code>int g() { int a = 5; int b = 5; return shift(42,a-b); } int h(int k) { return shift(42,k*0); } </code></pre> <p>For the case when the shift value is unknowable at compile time ...</p> <pre><code>int egad(int m, int n) { return shift(42,m-n); } </code></pre> <p>... the shift cannot be avoided ...</p> <pre><code># -- Begin _Z4egadii # mark_begin; .align 16,0x90 .globl _Z4egadii _Z4egadii: # parameter 1: 4 + %esp # parameter 2: 8 + %esp ..B1.1: # Preds ..B1.0 movl 4(%esp), %ecx #20.5 subl 8(%esp), %ecx #21.21 movl $42, %eax #21.10 shrl %cl, %eax #21.10 ret #21.10 .align 16,0x90 # LOE # mark_end; </code></pre> <p>... but at least it's inlined so there's no call overhead.</p> <p>Bonus assembly: volatile is expensive. The source ...</p> <pre><code>int g() { int a = 5; volatile int b = 5; return shift(42,a-b); } </code></pre> <p>... instead of a no-op, compiles to ...</p> <pre><code>..B3.1: # Preds ..B3.0 pushl %esi #10.9 movl $5, (%esp) #12.18 movl (%esp), %ecx #13.21 negl %ecx #13.21 addl $5, %ecx #13.21 movl $42, %eax #13.10 shrl %cl, %eax #13.10 popl %ecx #13.10 ret #13.10 .align 16,0x90 # LOE # mark_end; </code></pre> <p>... so if you're working on a machine where values you push on the stack might not be the same when you pop them, well, this missed optimization is likely the least of your troubles.</p>

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