Browse code
Add gcc-optimizations/avoid-imul.c.
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new file mode 100644 |
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+/* |
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+ * A few dummy functions to show how gcc avoids integer |
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+ * multiplication (i.e. the "imul" instruction) even with -O0. |
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+ * Of course, the avoidance is more, err, straightforward |
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+ * with -O1 and friends. |
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+ * Tested with x86-64 gcc 6.2 and -O0. |
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+ * |
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+ * Let's be honest, I was procrastinating something here... |
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+ */ |
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+#include <stdint.h> |
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+ |
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+uint32_t gcc_O0_optim_mul_2() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do multiplize_me + multiplize_me |
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+ return multiplize_me * 2; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_3() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do multiplize_me + multiplize_me + multiplize_me |
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+ return multiplize_me * 3; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_4() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do multiplize_me << 2 |
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+ return multiplize_me * 4; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_5() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do (multiplize_me << 2) + multiplize_me |
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+ return multiplize_me * 5; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_6() {
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+ uint32_t multiplize_me = 42; |
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+ // This will do gcc_O0_optim_mul_3 followed by gcc_O0_optim_mul_2 |
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+ return multiplize_me * 6; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_7() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do (multiplize_me << 3) - multiplize_me |
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+ return multiplize_me * 7; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_8() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do multiplize_me << 3 |
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+ return multiplize_me * 8; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_9() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do (multiplize_me << 3) + multiplize_me |
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+ return multiplize_me * 9; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_10() {
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+ uint32_t multiplize_me = 42; |
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+ // This will do gcc_O0_optim_mul_5 followed by gcc_O0_optim_mul_2 |
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+ return multiplize_me * 10; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_11() {
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+ uint32_t multiplize_me = 42; |
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+ // This will do gcc_O0_optim_mul_10 then add multiplize_me |
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+ return multiplize_me * 11; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_12() {
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+ uint32_t multiplize_me = 42; |
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+ // This will do gcc_O0_optim_mul_3 followed by gcc_O0_optim_mul_4 |
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+ return multiplize_me * 12; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_13() {
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+ uint32_t multiplize_me = 42; |
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+ // This will do gcc_O0_optim_mul_12 then add multiplize_me |
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+ return multiplize_me * 13; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_14() {
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+ uint32_t multiplize_me = 42; |
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+ // This will, at last, call imul |
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+ return multiplize_me * 14; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_15() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do (multiplize_me << 4) - multiplize_me |
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+ return multiplize_me * 15; |
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+} |
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+ |
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+uint32_t gcc_O0_optim_mul_16() {
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+ uint32_t multiplize_me = 42; |
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+ // This will actually do multiplize_me << 4 |
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+ return multiplize_me * 16; |
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+} |
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\ No newline at end of file |