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middle-end: Improve RTL expansion in expand_mul_overflow,

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This patch improves the RTL that the middle-end generates for testing
signed overflow following a widening multiplication.  During this
expansion the middle-end generates a truncation which can get used
multiple times.  Placing this intermediate value in a pseudo register
reduces the amount of code generated on platforms where this truncation
requires an explicit instruction.

2020-07-11  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog:
	* internal-fn.c (expand_mul_overflow): When checking for signed
	overflow from a widening multiplication, we access the truncated
	lowpart RES twice, so keep this value in a pseudo register.
This commit is contained in:
Roger Sayle 2020-07-11 20:03:39 +01:00
parent 90efb2512d
commit 2b4bbc1707
1 changed files with 3 additions and 0 deletions
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gcc/internal-fn.c
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@ -1629,6 +1629,9 @@ expand_mul_overflow (location_t loc, tree lhs, tree arg0, tree arg1,
profile_probability::very_likely ());
else
{
/* RES is used more than once, place it in a pseudo. */
res = force_reg (mode, res);
rtx signbit = expand_shift (RSHIFT_EXPR, mode, res, prec - 1,
NULL_RTX, 0);
/* RES is low half of the double width result, HIPART